Explanation:
let's just write the equations on top of each other as in a large subtraction, then transform one of them so that at least one term is the range in both equations, and then do the subtraction.
a)
I automatically write rational numbers as the total fractions (14 1/4 = 57/4, 3 3/4 = 15/4)
15x - 7y = 57/4
- 5x - y = 15/4
15x - 7y = 57/4
- 15x - 3y = 45/4
-------------------------
0 -4y = 12/4
y = -(12/4)/4 = -12/16 = -3/4
then we use that in the simpler second equation
5x - -3/4 = 15/4
5x + 3/4 = 15/4
5x = 12/4
x = (12/4)/5 = 12/20 = 3/5
b)
we don't need to transform any equation, as the x terms are already equal.
3x - 4y = 2
- 3x - 7y = -1
-------------------
0 3y = 3
y = 3/3 = 1
using this in e.g. the first equation
3x - 4×1 = 2
3x - 4 = 2
3x = 6
x = 6/3 = 2