Question

3/a^2 + 4/a^2 - 2/a^3

Answer 1

7-2/a^2-a^3 ??

Answer 2

In order to add/subtract fractions with variables, you need to treat them as you would normal numbers. The denominators must be the same, and you're only allowed to change things by multiplying/dividing the top and number by the same value (since x/x = 1 and anything times 1 is itself).

3/a² + 4/a² - 2/a³
Times first two expressions by a.
3a/a³ + 4a/a³ - 2/a³
Same thing as:
(3a + 4a - 2)/a³
Simplify.
(7a - 2)/a³