How do you add (5.4xx10^6) + (12.5xx10^2)?

The answer is $5.40125 \times {10}^{6}$.
To change the exponent on $12.5 \times {10}^{2}$ from 2 to 6, move the decimal to the left 4 places so the number is now $0.00125 \times {10}^{6}$, and now you can add the two numbers.
$5.4 \times {10}^{6} + 0.00125 \times {10}^{6} = 5.40125 \times {10}^{6}$*