Answer:
-3
Step-by-step explanation:
When you have an equation in the form y=mx+b, then the number at the end is the y-intercept.
I know this is not an answer but, I love your handwriting!
It is soooo much neater than mine!
- Laura <3
X+y=57
y=x+15
x+x+15=57
2x=42
x=21
y=21+15
y=36