How about you keep the roots in that form instead of making them into decimals?

(x - (4 + sqrt(5)))(x - (4 - sqrt(5)))

If you expand it out / consider Vieta's, you'll get x^2 + bx + c, where:

- b will be the negation of the sum of the roots ( -(r1 + r2) )

- c will be the product of the roots ( r1r2 ).

4 + sqrt(5) and 4 - sqrt(5) are pretty straightforward to add, and also to multiply, especially if you can recognize the fact that they're conjugates ( (a + b)(a - b) = a^2 - b^(2) ).

Once you get some basic parabola, you should be able to scale it by some factor (the other comments address this) so that substituting x gives you -12.