3x - 4a + 5a - 3x + a = ?

let's rearrange the numbers so that we have like variables beside each other, we can do that since addition is commutative

3x - 4a + 5a - 3x + a = 3x - 3x - 4a + 5a + a ................now add all the x's and a's together

.............................= 0x + 2a

.............................= 2a