This is not a Magic Formula:
Using the principles of co-ordinate geometry, we get:
(C - 0)/(F - 32) = (100 - 0)/(212 - 32)
==> F - 32 = (180/100)*C
==> F = (9/5)*C + 32
Example: Given that C = 100
==> F = (9/5)*100 + 32
==> F = 180 + 32 = 212 (ok!).