The coefficient 6.3 rounds to 6
The coefficient 9.9 rounds to 10
So we go from this
(6.3 x 10^(-2))(9.9 x 10^(-3))
To this
(6 x 10^(-2))(10 x 10^(-3))
Multiply out the coefficients to get 6*10 = 60
Add the exponents to get -2 plus -3 = -5
So we end up with 60 x 10^(-5)
Be careful. This is NOT the final answer. We need to convert this to scientific notation
60 = 6*10 = 6 x 10^1
60 x 10^(-5) = ( 6 ) x 10^(-5)
60 x 10^(-5) = ( 6 x 10^1 ) x 10^(-5)
60 x 10^(-5) = 6 x (10^1 * 10^(-5)
60 x 10^(-5) = 6 x 10^(1+(-5))
60 x 10^(-5) = 6 x 10^(-4)
The final answer is choice A