The difference between a number and 12 is 20

write an equation

Assume that this number is n.
Difference between two number means the subtraction between them.
Therefore, "The difference between a number and 12 is 20" means that when subtracting these two numbers, the result is 20.

Since the question did not mention which number is greater, therefore, we have two ways to write the equation. 
These two ways are as follows:
x - 12 = 20, and
12 - x = 20