Question

The difference of a number m and 30 is 10

Answer 1

m - 30=10

Difference is used as subtraction so you then identify that the word (is) is used as = so you create the equation around what you know.

Answer 2

Well, i'm assuming m is 40 since 30 + 10 = 40.
As an equation it is:
m - 30 = 10