Find the values of $\displaystyle x$ for all points on the graph of $\displaystyle f(x)= x^3 + 2x^2 + 5x -16$ at which the slope of the tangent line is5.

my work:

$\displaystyle f(x)= x^3 + 2x^2 + 5x -16$

$\displaystyle f`(x) = 3x^2 + 4x + 5$

$\displaystyle 5 = 3x^2 + 4x + 5$

$\displaystyle 0 = 3x^2 + 4x$

stuck after this...