If $3+a=4-b$ and $4+b=7+a$, what is $3-a$?
One way to solve such a system is to use substitution or elimination. I will use substitution, since it looks easier to isolate one variable in each equation. From the first equation, I can solve for $a$ in terms of $b$: $a=4-b-3=1-b$. Then I can substitute this expression for $a$ into the second equation: $4+b=7+(1-b)$. Simplifying this equation, I get $4+b=8-b$, or $2b=4$. Dividing by 2, I get $b=2$. Now that I know $b$, I can plug it back into the expression for $a$: $a=1-b=1-2=-1$. The question asks for $3-a$, so I just need to substitute $a=-1$ into this expression: $3-a=3-(-1)=3+1=4$.

