You want to get rid of the plus 17 on the left side of the equation, so because it is attached to 2x by addition you have to subtract it and do the opposite.
2x + 17 = -3
<u> -17 -17</u>
2x = -20
-3 - 17 is equal to -20, so you are left with 2x = -20. Then because 2 is attached to x by multiplication, you have to divide 2 on both sides to get x. -20 divided by 2 is -10, so the answer is A.