You can see from the image that you intersect the circle of radio range in two spots.

Let the tower be at (0,0). City 1 is then at (0,60) and city 2 at (70,0).

Consider the line connecting the two cities.

The slope of this line is $m=\dfrac{(0-70)}{(60-0)}=- \dfrac 7 6$

The equation of the line is thus

$(y-60) = -\dfrac 7 6 x \Rightarrow y = -\dfrac 7 6 x$

In the quadrant where the trip takes place the equation for the circle of radio range is

$y = \sqrt{50^2 - x^2}=\sqrt{2500 - x^2}$

Now just solve for the two intersections

$\sqrt{2500 - x^2}=-\dfrac 7 6 x$

can you do this? You'l get two solutions corresponding to the two point on the circle. Find the distance between them. Find the distance between the cities. Take the ratio of these two distances and you're done.

The distance between two points is $\sqrt{(x_2-x_1)^2 + (y_2-y_1)^2}$