You can put this solution on YOUR website! If you make a right triangle diagram of the ramp, the height is 84 feet, and you wish to find the hypotenuse. Looking at the angle of elevation, the sides in consideration are the opposite side and the hypotenuse, suggesting the sine relationship.
sin x = (opposite) / (hypotenuse)
sin 20 = 84 / length of ramp
Now solving we get
length of ramp = 84 / sin 20 = 84 /.342 = 245.6 feet