How do you work out this maths question

joe and jacob thin of the same number.
joe doubles his number and subtracts 13.
jacob subtracts 4 from his number and multiplies by 3.
They both get the same answer. What number did they think of?

Joe and Jacob are thinking the same number, lets say x
Joe doubles his numbers, 2x, and then subtracts 13, 2x-13.

Jacob subtract 4 from his number, x-4, then multiplies by 3, 3x-12

Set them equal to each other.

2x-13=3x-12
-13=x-12
-1=x

The number they are both thinking of is -1.

Let x be joes and jacobs number:

joe doubles his number, and subtracts 13, so:
2x - 13

Jacob subtracts 4 and multiplies by 3, so:

3(x - 4)

Now since these are equal, make them equal each other, then solve for x:

2x - 13 = 3x - 12

x = -1

make it into an equation:
2x-13=3(x-4)
2x-13=3x-12 add 12 on both sides , minus 2x on both sides
-1=x
and thats the number theyre thinking of ....
2(-1)-13=15
3(-1-4)=15

2x - 13 = (x - 4) * 3
2x - 13 = 3x - 12
-13 + 12 = 3x - 2x
-1 = x