The answer would be:
20.9 ft
Here is why:
If you draw the scenario, (which is attached below) you can see that a right triangle is formed. The ramp length is the hypotenuse of the scenario. To solve for it, we can use the Pythagorean Theorem where:

Where:
c = hypotenuse (Longest side)
a and b = legs of the triangle
Let's take our given and put it into the formula:
c = length of the ramp
a = 6ft
b = 20ft





Hope you get it!