A quadratic with roots at 8 and 5 is f(x) = x^2 + 10x - 24
In order to find an equation given roots you can create statements that equal 0 in order to create parenthesis. For instance we know x = 8 at one point. So, we can solve that to equal 0.
x = -12 ----> add 8 from both sides
x + 12 = 0
We can do the same for the other zero.
x = 2 ----> subtract from both sides
x - 2 = 0
Now that we have both of these, we can multiply these two things together. This will give us the function we need.
f(x) = (x + 12)(x - 2)
f(x) = x^2 + 12x - 2x - 24
f(x) = x^2 + 10x - 24