2(a-4)+15=13

Hi anonymous.

Thanks for joining in.  

You were right up to 

a-4=-1

now add 4 to both sides

a-4+4=-1+4

-1,0,1,2,3,4,5,  They are the numbers on my invisable number line.  Start at -1 and go 4 places in the positive direction.   You will get to  -1,0,1,2,3,    

a=3

2(a-4) + 15 = 13

Distribute the 2:

2a - 8 + 15 = 13

Collect "like" terms:

2a + 7 = 13

Subtract 7 from both sides:

2a = 6

Divide by 2:

a = 6/2 = 3.

a = 3.

2(a-4)+15=13    simplify on the left

2a - 8 + 15 = 13

2a + 7 = 13   subtract 7 from both sides

2a = 6   divide by 2 on both sides

a = 3

Check

2(3-4) + 15 = 13

-2 + 15 = 13

13 = 13

2(a-4)+15=13

2(a-4)=-2

/2

a-4=-1

a=-5