To do the sum of two numbers in scientific notation we must put them in the same exponent, therefore, if we want to sum

We must put both coefficients in 2 or 3, we can choose, I'll put the 2.8 in exponent 2, therefore

Now we can preserve the exponent and sum the coefficient

Therefore we have

But see that the result is not in scientific notation, so let's adjust it:

Therefore, the result in scientific notation is

The coefficient (green) is 3.53 and the exponent (yellow) is 3