Assuming the house is parallel to the ground, your triangle is made up by the house, the ground, and the ladder. Using sin(x) = opposite/hypotenuse on the angle 70 degrees, you would have sin(70) = 20/x (because 20 is the side opposite of the angle connecting the ladder and the ground)
--> simplify sin(70) * x = 20 --> x = 20 / sin(70
--> substitute (and make sure your calculator in in degrees) x= 21. 284