5a + b = 14
You need to get a by itself on one side of the equation, by doing the same thing to both sides. The last operation that is done on the left side of the equation is adding b, so undo that by subtracting b from both sides.
(5a + b) - b = 14 - b
5a = 14 - b
Now the last operation done on the left is multiplying by 5, so undo that by dividing both sides by 5.
(5a) / 5 = (14 - b) / 5
a = (14 - b) / 5
