What is the average rate of change over to interval 1 ≤x≤3​

the difference of two squares is a special form that has a factorization worth memorizing.

a² - b² = (a - b)(a + b)

your equation can be factored as

x² - 16 = 0

(x - 4)(x + 4) = 0

the zero product rule tells you the solutions are the values of x that make these factors zero: x=4, x=-4.