Cynthia, we can use the law of sines with what is given and what we can infer.
We have an unkown side length opposite the 70 degree angle. We can assume that the angle between the ground and the building is 90 degrees, so the angle between the building and beam of light (opposite the ground) is 20 degrees.
sin20/240 = sin70/x
xsin20 = 240sin70
x = 240sin70/sin20
I don't have a calculator handy, but plug it in to yours for your answer!