The average velocity is the gradient of the line that joins and .
The instantaneous velocity is the gradient of a line that joins any point to another point extremely close to it. So for any point , you can say that a point close to it is , and the gradient would be .
So in your question, to evaluate the instantaneous velocity at , you know that on the curve, when . A point close to this would be , so .
Simplify then put into your equation for the gradient.