Determine x-intercept of function. $\displaystyle f(x) = x^2 -8x -18 $
I know I need to set to zero.$\displaystyle x^2 -8x -18 =0 $
Isolate for x. $\displaystyle x(x-8) -18 =0 $
Indeed you do set it to 0 to get it in the form $\displaystyle ax^2+bx+c=0$.
Since $\displaystyle a$ and $\displaystyle c$ have different sign then there will be two real roots (ie: $\displaystyle b^2-4ac>0$ which is what mathfun showed). This won't factor (136 is not square) and so use the quadratic formula.
For the purposes of simplifying the square root: $\displaystyle 136 = 2^3 \cdot 17$