You got the volume so the formula for finding a rectangular prism is l*w*h=x. l stands for length, w stands for width, h stand for height, and x stands for the total volume. Put the numbers in the formula so do 3*5*l=45 multiply the 3 and 5 and you will get 15*l=45 and divide by 15 on both sides (use a calculator if you can't do it in your head) and the answer should be 3.