Question

What's the product of 21 and (6.3 times 10-4)

Answer 1

21(6.3 x 10 - 4).  Remember PEMDAS.  Everything done in parenthesis is done first.

Within the parenthesis, there is multiplication and subtraction.  In PEMDAS, multiplication/division comes before addition/subtraction, so 6.3 x 10 = 63.
63 - 4 = 59
21(59) = 1,239.