We can solve for a variable in an equation by isolating on one side of the equation and getting everything else on the opposite side.
a + 6 = 3a + 2
The variable a is on both sides. Get them on one side.
a + 6 = 3a + 2
6 = 2a + 2 <-- Subtract a from each side
Now the variable a is only on one side. Undo any operations done to a by using the inverse of those operations
6 = 2a + 2
The variable a is being multiplied by 2 and added to 2. Undo all of those operations.
6 = 2a + 2
4 = 2a <-- Subtract both sides by 2
2 = a <-- Divide both sides by 2
So, a is equal to 2.