$$ \begin{aligned} \int e^{\sec x}(\sec x \tan x f(x)+(\operatorname{sex} x \tan x & \left.\left.+3 e^2 x\right)\right) d x \\ & =e^{\sec x} f(x)+C \end{aligned} $$ $\therefore \quad$ We know that $$ \begin{array}{ll} \int e^{g(x)} & \left(\left(g^{\prime}(x) f(x)\right)+f^{\prime}(x)\right) d x=e^{g(x)} \times f(x)+C \\ \therefore & f(x)=\int\left((\sec x \tan x)+\sec ^2 x\right) d x \\ \therefore & f(x)=\sec x+\tan x+C \end{array} $$