The given expression is
To add 2 fractions we have to give them the same denominators
In the up part (numerator) we have 2 different denominators a and b
Then the common denominator of the 2 fractions must be the product of them
Then the fraction will be
Now, we will multiply the denominator of up by the denominator of the first fraction (ab x b)
Multiply b by b to simplify, then
The numerator is 2b + 7a
The denominator is ab^2