Step-by-step explanation:
just adding +y on both sides of 1st equation
x+y = 12
now multiplying the whole 1st equation with 2
2x+2y=24
now subtract the equation 1 from equation 2
2x+3y = 29
2x+2y = 24
----------------
y = 5
use value of y in any equation
x+5 = 12
x = 7
y = 5
the above values satisfy both equations.
i hope this helps
