How to factor out tue coefficient of the variable

Lets say you have a question such as 7x+1=15.

First you have to start with the addition or subtraction because of PEMDAS. In order to get rid of the +1, you have to subtract 1 from it and do the same thing to the number on the other side of the equal sign.
7x+1-1=15-1
Which leaves you with
7x=14
What you want to do is get the variable by itself so, since the variable is basically multiplying itself by the coefficient, you want to divide the 7x by 7 to get rid of the coefficient. Make sure to do the same for both sides of the equation!
7x/7=14/7

x=2

Thats the answer.
Hopefully you can figure this out on your own problem.

38+105= 143, 39+104=143, and you will find then adding the numbers on opposite sides, moving inwards yields the answer 138 every time. because of this, you’re going to multiply 138 times the number of pairs, which is 34. therefore, 34*138= 4692(i’m sorry if it’s the wrong numbers, but it’s the right concept)