23.
By completing the square, solve
x^2 + 12x -c=0
where c is a positive constant.
Give your answers in surd form in terms of c.
You must show your working.

To find the intersection points, you take x^2 - 4x + 9 = -2x + 17

This gives x^2 - 2x - 8 = 0,

which factorises to (x - 4)(x + 2) = 0, giving the solutions x = 4 or x = -2

From the graph, we can see that x is positive, meaning x = 4, so a = 4.

Substituting x = 4 into y = -2x + 17, we get y = -2(4) + 17 = 9, so b = 9.

a + b = 4 + 9 = 13. The answer is B.