3x + 2y = 5a + b (1)
4x - 3y = a + 7b (2)
I think that the shortest (the most straightforward) way to get the goal is to eliminate "y".
For it, multiply eq(1) by 3 (both sides) and multiply eq(2) by 2. You will get
9x + 6y = 15a + 3b (3)
8x - 6y = 2a + 14b (4)
Now add eq(4) and eq(3). You will get
17x = 17a + 17b ========> x = a + b. ANSWER