first you need the value of A:
6 in for the height, 8 in for the width (i'm sorry, i don't know how to spell it) and 15 for the lenght
6*8*15 = 720 in³
the measurements are almost the same for the other except the width
B: 6*16*15=1440
and C:
6*24*15 = 2160
add this up and the value of the composite figure is 4320 in³