Someone help I will make you

What number should be added to both sides of the equation to complete this square? x^2-10x=7

Let f(x) = 3 − x . find the average rate of change of f(x) from x = a to x = a + h and simplify your answer so that no single factor of h is left in the denominator.

Iknow its nothing to do with education, but what type of thing should i wrote here? ? : ) its for mcdonald's btw.