In order to find the length of the rectangular prism, take into account the following formula for the volume of the prism:
V = l · w · h
where, in this case, you have:
l: length = ?
w: width = 4 in
h: height = 3 in
V: volume = 60 in³
Solve for l in the formula for V, by dividing by w·h both sides
V/(w·h) = l
l = V/(w·h)
replace the values of the parameters to obtain l:
l = (60 in³)/(4 in · 3 in ) = 5 in
Hence, the length of the rectangular prism is 5 in