F[C_]:=1.8*C+32
F[20]

p1=Plot[F[C],{C,-40,40}]

Ce[F_]:=(F-32)/1.8
Ce[-40]

p2=Plot[Ce[F],{F,-40,40}]

Show[{p1,p2},AspectRatio->2]

g[x_]:=x^3+x-2;
Plot[g[x],{x,-4,4}]