How do I simplify the expression like the question ask

The following is Mobius' explanation for a solution to this question. You can use this and other online references as a guide, but your explanation should be in your own words. The derivative of a function of f at x is given by f' (x) = lim f(xth)-f(x) h-0 h provided the limit exists. Use the definition of the derivative to find the derivative of f (a) = 6x2 + 9x + 5. Enter the fully simplified expression for f (x + h) - f (x). Do not factor. Make sure there is a space between variables. f (x + h) - I(x) = f (x) = 12x+9