The functions f(x) and g(x) satisfy f(0) = 3, g(0) = -4, and
f'(x) = 7f(x) + 2g(x),
g'(x) = -4f(x) + g(x).
Find f(x).
$f(x) = 2e^{2x} - 5e^{3x}$.
