2x = -6 is an equation, that is both sides have the same value. Thats why you have the "equal to" sign between them.
If 2x = -6,
then whatever you do to the left hand side, you must also do to the right hand side.
Divide the left hand side by 2, (so that the x variable would be isolated), and at the same time divide the right hand side by 2 also (remember that its an "equation" they're both equal so you must carry out the same operation on both of them).
2x/2 = -6/2
x = -3