Answer:
Equation:

Base: 60 feet.
Explanation:
The formula for calculate the area of a triangle is:

Where:
A: is the area of the triangle.
b: is the base of the triangle.
h: is the heigth of the triangle.
Knowing the area of the triangle and its height, you can write the equation:

Where b is the base in feet of the A-frame house.
To solve it:
Multiply both sides of the equation by 2.
Divide both sides of the equation by 20.
Then:

The base of the A-frame house is 60 feet.