You need to calculate the following expression:
f(b) - f(a) [(10)^2-3(10)-4]-[(7)^2-3(7) -4]
--------------- , which in this case is: ------------------------------------------
b - a 10-7
Be sure to do the calculations inside each set of square brackets separately BEFORE attempting the subtraction in [(10)^2-3(10)-4] - [(7)^2-3(7) -4]
By the way: You mean "average rate of change," not "average of change."