Answer
Odd: $b=0$.
Even: $m=0$.
Work Step by Step
We know that if a function is odd, then $f(-x)=-f(x).$
We know that if a function is even, then $f(-x)=f(x).$
Hence we plug in $-x$ to see what happens.
$f(-x)=-mx+b.$
If it is odd, then $-mx+b=-f(x)=-(mx+b)\\-mx+b=-mx-b\\b=-b\\2b=0\\b=0$
If it is even, then $-mx+b=f(x)=mx+b\\-mx+b=mx+b\\-mx=mx\\mx=0\\m=0.$