Answer
False
Work Step by Step
You can see this by subbing it into an equation. lets suppose that f(x) = $x^{2}$
and our value for are variable are s = 2 and t = 5. when we sub this in we get
$7^2$ = $2^2 + 5^2$
49 = 4 + 25
this statement is false meaning that
f(s+t) $\ne$ f(s) + f(t)
