### Instruction: If $3+a=4-b$ and $4+b=7+a$, what is $3-a$?### Response: This problem involves two linear equations with two variables, $a$ and $b$.
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 $a$: $a=1-b=1-2=-1$.