We know the angle of depression is x, so using alternate interior angles, the angle at the ground is also x
We can either use trig functions or the Pythagorean theorem to find d
I like to use the Pythagorean theorem
a^2 + b^2 = c^2 where a and b are the legs and c is the hypotenuse
7^2 + d^2 = 20^2
49 + d^2 = 400
d^2 = 400-49
d^2 = 351
Taking the square root of each side
d = sqrt(351)
d =18.734994
To the nearest tenth
d = 18.7 yds