21) (...+0,2h)² = 25d² + ... + ...

The expression is of the form (a+b)², where b=0.2h and a²=25d². Thus, a=5d. (5d+0.2h)² = (5d)² + 2(5d)(0.2h) + (0.2h)² = 25d² + 2dh + 0.04h². The missing terms are 5d and 2dh.