3 Use the integrating factor method to find the solution of the differential equation $$\frac { \mathrm { d } y } { \mathrm {~d} x } + 2 y = \mathrm { e } ^ { - 3 x }$$ for which \(y = 1\) when \(x = 0\). Express your answer in the form \(y = \mathrm { f } ( x )\).