When adding and subtracting fractions, you need a common denominator.
In this one, you already have the common denominator,
so just combine the numerators and put over the common denominator
(2a + 8)/(a + 17) - (a - 9)/(a + 17)
(2a + 8) - (a - 9)
------------------
a + 17
2a + 8 - a + 9
--------------- {took out parentheses in numerator}
a + 17
a + 17
------- {combined like terms in numerator}
a + 17
