Let:
y=15+3x

y=27-3x

Add each side to eliminate 3x and solve for y: 2y=42

y=21

Solve for x using (1)

21=15+3x

6=3x

2=x

So, the solution of the system is: (2,21)

y=27-3x

Add each side to eliminate 3x and solve for y: 2y=42

y=21

Solve for x using (1)

21=15+3x

6=3x

2=x

So, the solution of the system is: (2,21)