Let $\displaystyle f_1(x)$ and $\displaystyle f_2(x)$ be odd and even functions respectively. How can we construct an even function out of these?
Plenty of ways, but why can't you just take $\displaystyle f_2(x)$ as the even function? What's the point of using $\displaystyle f_1(x)$ in constructing an even function?
Edit: In case you mean you need to construct an even function from only $\displaystyle f_1(x)$, some simple ways would be to take the absolute value or square the function.
$\displaystyle g(x)=|f_1(x)|$
$\displaystyle h(x)=(f_1(x))^2$
These would both be even functions.