First, find the height of the entire pyramid. The subtract the volume of the upper pyramid from the volume of the entire pyramid.
Draw the frustum and extend the lines up to complete the entire pyramid. Using similar triangles, you can find that the height of the entire pyramid is 30 in.
volume of entire pyramid = (1/3)Ah = (1/3)(5 in)^2 * 30 in = 250 in.^3
volume of upper pyramid = (1/3)Ah = (1/3)(3 in)^2 * 18 in = 54 in^3
volume of frustum = vol of entire pyramid - vol of upper pyramid
volume of frustum = 250 in^3 - 54 in^3 = 196 in^3
