Let f(x)=x^2 - 3x - 4 . what is the average of change from x=7 to x=10?

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."

Where’s the problems ? ?