If you form a triangle with the points B and C as two of the points of the right angled triangle then you use Pythagoras to find the length of BC.
3^2+4^2=25
√25 = 5
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To determine the number of cubes he needs to fill the box (this is assuming the cubes are 1 in cubes, he would need to calculate the volume of the box. To find the volume he would multiply the length by the width by the height. This would be 5 in x 6 in x 7 in. The volume is 210 cubic inches, so he could fill it with 210 one inch cubes.
15!!! is the answer to your question:)