To solve for x, you must use inverse operations.
We must move the 12 from the right, to the left in order to solve for x.
The inverse of addition, is subtraction.
Subtract 12 from both sides
7 - 12 = x + 12 - 12
To subtract a bigger number from a smaller number, flip it around, and make your answer negative
12 - 7 = 5
7 - 12 = -5
Something minus itself is 0
-5 = x + 12 - 12
-5 = x + 0
We like to have x on the left, and the = is symetrical. Just flip it around
x = -5 + 0
x = -5
Verify
7 = -5 + 12
7 = 12 - 5
7 = 7
true