f(x)=x/(x+5) is shown in part as the red curve for x ∈ (-5,5].
But if we take the interval [0,5] and reflect it in the y axis (blue curve) we can convert it into an even function, such that [-5,0] is a reflection of [0,5].
The rule for even functions is that f(x)=f(-x).
Therefore f(-3)=f(3)=⅜.