Question 21838
When you have the common denominator of 90ab^2 as the 
denominator of your two fractions (which you did 
correctly)  your next step is to add the numerators
of the two fractions and write that sum over the common
denominator, as follows:

   [42a^2+5b^2)/[90ab^2]
That is as far as you can go because you have on
"like" terms in the numerator and no common factor.
You cannot add the 42 and the 5 because the terms
are not "like"---it is like 42 apples and 5 oranges,
which cannot be added.

Cheers,
Stan H.