Answer :
f(x)=3^x
because: when x=1; f(x)=3^1=3 (pair (1;3)
x=2; f(x)3^2=9
x=3; f(x)=3^3=27
x=4; f(x)=3^4=81
because: when x=1; f(x)=3^1=3 (pair (1;3)
x=2; f(x)3^2=9
x=3; f(x)=3^3=27
x=4; f(x)=3^4=81