Question: this is the solution to the augmented matrix with the...

This is the solution to the augmented matrix with the two variables a and b. I am having trouble understanding why and how they got -3/2a + b in the second step. Why did they add a to the b in the bottom row? Should a have just stayed up top? If you could please provide detailed solution with the algebra they did for this question/solution, that would be very helpful.