F(3)=-18
You just replace x for three for your equation and then use Ofer of operations to solve it.
So f(3)= 21+ 2(3)-5(3)^2
The exponent goes first: f(3)= 21+ 2(3)-5(9)
Next is multiplication: f(3)= 21+ 6-45
Now add: f(3)= 27-45
Finally subtract: f(3)= -18
Hope this helps!
