PLEASE HELP OR I WILL FAIL

Given that (x-3) is a factor of f(x)=x^3+ax^2-x-6, find a and hence the number of real roots or the equation f(x)=0

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

Atriangle has an area of 72 square inches. if the base of the triangle has a length of 18 inches, what is the height of the triangle? use the formula for the area of a triangle: area = (base)(height) type a numerical answer in the space provided. do not include the units or spaces in your answer.