How do I solve this this question

to find the sum:
2(3t-4) - (t^2 + 2) - 4t(t-1)
6t - 8 - t^2 - 2 - 4t^2 + 4
6t - 8 - 5t^2 + 2
-5t^2 + 6t - 6
by the sum of roots and product of roots formulae
ax^2 + bx + c
x + y = [tex] \frac{-6}{-5} [/tex]
x + y = [tex] \frac{6}{5} [/tex]
xy = [tex] \frac{-6}{-5} [/tex]
xy = [tex] \frac{6}{5} [/tex]
x - y =