There are two sites of every equation. The sign = means that the two sites are equal.
So, we know that 3x+7 is equal as 13. 3x+7=13
In order to find the unknown x we should rewrite the equation such that on one site we have only x and on the other all left
3x=13-7 ("+"is "-" on the other site of the equation)
3x=6
x multiplied with 3 gives 6, so we need to divide 6 with 3 in order to get the value of x.
x=6/3=2