Question

how to get the answer 900 square ft3 using 6cm3 and 15cm3

Answer 1

I have no idea. It looks like typos or missing information.

Answer 2

assuming you meant 900 cm^3, then
50*6cm^3 + 40*15cm^3 = 900 cm^3

There's no such thing as "square ft^3"

However, 10 * 6cm^3 * 15cm^3 = 90 (cm^3)^2