First, you make them have similar exponents by changing one to the other then changing the base.
For example: 6.8x10^2 + 2.1x10^3
or 0.68x10^3 + 2.1x10^3
In this example, you will raise the exponent by one, therefore, you have to divide the base by 10 , changing its exponent but not the value.
The answer is 2.78x10^3.
After that, you just simplify the base to the least amount of significant figures. In this example, there are 2 so the answer is 2.8x10^3.