In order to sum 9.72 x 10⁸ + 1.93 x 10⁷, consider that it is necessary that both 10 factors have the same exponent.
You can increase the exponent of the second number from 7 to 8, by moving the decimal point one unit to the left, just as follow:
1.93 x 10⁷ = 0.193 x 10⁸
now, you can sum the given numbers:
9.72 x 10⁸ + 0.193 x 10⁸ = 9.913 x 10⁸
where you have added the coefficients of the previous terms.