Find the primitive of 2x, i.e. what should I differentiate to get 2x?
The answer is obviously x^2. (The general rule of thumb for the antiderivative of x^n is (x^(n+1))/(n+1) as long as n is not -1)
Then you substitute each of the threshold values (10 and 13) into this primitive and subtract. that is, your answer is
13^2 – 10^2
Strictly speaking, the antiderivative of 2x is x^2 + C, where C is an arbitrary constant, because the derivative of a constant is zero. However, when you’re going to substitute and subtract, there’s no point in including C, because
(13^2 + C -(10^2 + C)) is always 13^2 – 10^2.