What is the formula for area of triangle? A=1/2bh. So we do have the area and the height. So plug in what you have and solve for b. When you plug it in, you should get 150 in^2= 1/2 x b x 20. So solve 1/2 x 20, and that is 10. The remaining of the equation is 150 in^2=10b. In order to isolate b, divide 10 in both sides, and b should equal 15. :)