int (0 +00) a * e^(-2x) dx = a * int (0 +00) e^(-2x) dx =
= a * (-1/2 * e^(-2x))_{0}^{+00} = -1/2 * a * (e^(-2x))_{0}^{+00} =
= -1/2 * a * (lim (x->00) 1/e^(2x) - e^0) =
= -1/2 * a * (0 - 1) = 1/2 * a
int (0 +00) a * e^(-2x) dx = 1/2 * a = 1 => a = 2.