Okay, Dick's method works, but it took a while to see why.
You have two striaght line graphs plotting temperatures in degrees C & degrees F.
These intersect at -40.
Adding 40 to any quoted temperature moves the intersection point to Zero degrees.
Multiplying by 5/9 (or 9/5) gives the correction for the slope of the graph (1 in 100 or 1 in 180).
Subtract 40 to reset the intersection point of the graphs.

Much easier that the equation I learnt in school: (F-32)/9=C/5

Unfortunately I've had that equation in my mind for 40+ years, so it won't be easy to replace!