If f(x)=2x^2+1, what is f(x) when x=3?

Simply replace the variable by the value to evaluate the function at that point.

f(x)=2x^2+1, when x=3, means

f(3)=2(3^2)+1

f(3)=2(9)+1

f(3)=18+1

f(3)=19