it depends how complex your course is

the easiest way is to just consider two of the given points

instead of using (x,y) points

use (c,f) points and consider

Freezing point of water 0 32

Boiling point of water 100 212

so

(c1,f1) = (0,32) and (c2,f2) = (100,212)

gradient = m = (f2-f1)/(c2-c1) = 180/100 = 9/5

then use the point gradient formula for a line

f – f0 = m(c-c0)

take (c0,f0) = (0,32)

[[but you could use (100,212) instead if you like]]

so

f – 32 = (9/5)(c-0)

gives

f = (9/5)c + 32

or rearranging

c = (5/9)(f-32)

Which would I use for the x axis and which for y?

it doesn’t matter whether you use c for x and f for y or vice versa since you will get the same result either way just by rearranging the equation at the end

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