First you need to get x on one side of the equation and to do that subtract 2a from both sides.
28 + 2a = 5a + 7
-2a -2a
28=3a+7
Then we need to get a alone by subtracting 7 on both sides.
28=3a+7
-7 -7
21=3a
Finally divide each side by 3 and you should get a = 7. Hope this helps!