What is the average rate of change for f(x)=x^2 9x=18 from x= -10 to x=10

Average rate of change from x = -10 to x = 10 = (f(10) - f(-10))/(10 - (-10)) = (((10)^2 + 9(10) - 18) - ((-10)^2 + 9(-10) - 18))/(10 + 10) = ((100 + 90 - 18) - (100 - 90 - 18))/20 = (172 - (-8))/20 = (172 + 8)/20 = 180/20 = 9