Question

The formula to compute a person's body mass index is B = 700 x W/H2 (W/H2 means W/h squared, the h is squared)

Answer 1

By "solve" in part A, do you mean make W the subject of the formula? In which case:
A:  B = 700 x W/h2
     B x h2 =700 x W
     (B x h2)/700=W
B: (21.45 x 64^{2})/700=W
     87859.2/700=W
     126=W
     126 pounds