[tex]\huge{\boxed{f(4)=\bf{-3}}}[/tex]
In this case, you are replacing all instances of [tex]x[/tex] with [tex]4[/tex]. [tex]f(4)=3(4)-15[/tex]
Multiply. [tex]f(4)=12-15[/tex]
Subtract. [tex]f(4)=-3[/tex]
