You have the following algebraic expression:
7a² + 10a - 3a - 2a²
In order to simplify the previous expression, consider that you can only simplify like terms. Like terms are term with the same variable and same exponents.
Then, you have:
7a² + 10a - 3a - 2a² order like terms
7a² - 2a² + 10a - 3a simplify like terms
5a² + 7a
Hence, the simplified expression is 5a² + 7a