#### Answer

$0$

#### Work Step by Step

RECALL:
$f(a)$ represents the value of the function $f$ when $x=a$.
Thus, $f(3)$ represents the value of the function $f$ when $x=3$.
Substitute $3$ to $x$ in the function $f$ to obtain:
$f(3) = 3^2-5(3)+6
\\f(3)=9-15+6
\\f(3)=-6+6
\\f(3)=0$