(2a + a2) × 3 for a = 5?

Correct presentation is important in math. You can not let people interpret your statement in arbitrary ways. I have to guess at what you mean.

(2a + a^2) × 3 We use Shift-6 for powers. You have good use of parentheses, and good addition of spaces to improve readability.

First evaluate the multiplication.

6a + 3a^2

Now plug in the given value.

6 x 5 + 3 x 5^2

30 + 75 = 105 <-- ANSWER