The lines 3x + 4y = 12 and 3x + 4y = 72 are parallel. Find the distance that separates these lines.
I assume that you mean the perpendicular distance between the two parallel lines.
1. Line $\displaystyle l_1: 3x+4y=12$ has the y-intercept $\displaystyle P(0, 3)$.
Line $\displaystyle l_2: 3x+4y=72$ has the y-intercept $\displaystyle Q(0, 18)$.
2. The (perpendicular) distance of a point $\displaystyle P(p, q)$ to a straight line with the equation $\displaystyle Ax+By=C$ is calculated by
$\displaystyle d(P,l) = \dfrac{Ap+Bq-C}{\sqrt{A^2+B^2}}$
3. Calculate the distance of P(0, 3) to $\displaystyle l_2$:
$\displaystyle d(P,l_2)=\dfrac{3 \cdot 0 + 4 \cdot 3 - 72}{5} = -12$
4. So the distance you are looking for has the value 12. The negative sign of d indicates that the point P and the origin are on the same side of the straight line.
Oh, blast! I had just claimed that those lines are NOT parallel, then realized that the "3" multiplying x in the first equation, at least on my web viewer, had been separated by a line break!
Here's how I would do that- because I am terrible at memorizing formulas! The slope of the two lines is -3/4 so a line perpendicular to both will have slope 4/3. The equation of a line with slope 4/3 and passing through (0, 3) is $\displaystyle y= \frac{4}{3}x+ 3$. That will cross the second line where $\displaystyle 3x+ 4y= 3x+ 4(\frac{4}{3}x+ 3)$$\displaystyle = \3x+ \frac{16}{3}x+ 12= 72$. Then $\displaystyle (3+ \frac{16}{3})x= \frac{25}{3}x= 72- 12= 60$ and $\displaystyle x= \frac{3}{25}60= \frac{36}{5}$. And, of course, $\displaystyle y= \frac{4}{3}\frac{36}{5}+ 3= \frac{48}{5}+ 3$.
That is, the line perpendicular to both given lines intersects them at $\displaystyle \left(0, 3\right)$ and $\displaystyle \left(\frac{36}{5}, \frac{48}{5}+ 3\right)$. I have left the y value in that form because we need to subtract 3 from it to find the distance between those points.
The distance is $\displaystyle \sqrt{\left(\frac{36}{5}\right)^3+ \left(\frac{48}{5}\right)^2}= \sqrt{\frac{3600}{25}}= \sqrt{144}= 12$.
(Added after thinking it over) However, note Prove It's post. The first line passes through the point (0, 3) and the second through (0, 18). The difference between the two y intercepts is 18- 3= 15. Now that is NOT the distance between the lines (which is always measured perpendicular to both lines) but if we were to drop a perpendicular from, say, (0, 3) to the second line, it is the hypotenuse of a right triangle that has the common perpendicular to both lines as a leg. Further, a little geometry shows that those two legs in the ratio of 3 to 4 since the slope of both lines is -3/4. That is, if the perpendicular distance is "x" then the other leg has length (3/4)x so we must have $\displaystyle x^2+ 9x^2/16= 15^2$ or $\displaystyle \frac{25}{16}x^2= 15^2$..
$\displaystyle x^2= \frac{16}{25}15^2}= 144$.