If y = x^2 + 5x - 9,for what values of x is y>5?
To find range of x for which f(x)>5, where
f(x) = x^2 + 5x - 9
Solve for y=5, or
5 = x^2 + 5x - 9
x^2 + 5x - 14 = 0
(x-2)(x+7)=0
Since f(x) is U-shaped, the part of f(x) for -7<x<2 is below the line y=5.
So the part of f(x) above the horizontal line y=5, or f(x)>5 is (-∞-7)∪(2,∞).