Question

The difference between 10 and three times a number is 31.

Answer 1

10 -3h = 31

minus both sides by 10 to get:
-3h =21

divide both sides by -3 is:

h= -7

Answer 2

Let's write an equation describing this relationship:

let's call the number (n)
10 - 3n = 31
now we solve for 31
subtract 10 on both sides
10  - 3n - 10 = 31 - 10
-3n = 21
now divide by 3 on both sides
and we get 
n = -7