Tom want to reach a second layer floor window on a house that is 20 feet above ground, if he must put the ladder at 70 degree angle to the ground, how long must the ladder be?

It must be 21.3 feet.

The sides of the triangle given to us are the side opposite the angle to the ground, 20, and the missing side is the hypotenuse.  

opposite/hypotenuse is the ratio for sine:

sin 70 = 20/x

Multiply by x:
x sin 70 = 20

Divide both sides by sin 70:
(x sin 70)/(sin 70) = 20/(sin 70)
x = 21.283 ≈ 21.3