I suck at algebra but my advice is download photomath it will help trust me
Let

be the point on the ground to with the 18 foot guy is anchored and

the point on the ground to with the 21 foot guy is anchored.
We can conclude that the <span>the two wires at the points where they are anchored are
21 feet apart. Check the procedures in the picture attached.</span>
to find the slope for the first one, use the formula 
(It's the slope formula)
so... 
then plug in... 
this line has a slope of 0
As for the miles problem, to find mph, the miles have to be at 1.
so... 
therefore, he drove 60.5 mph