Symmetry with respect to the first bisector

Question

Knowing the points $(3m+n,2)$ and $(2m+n,-1)$ are symmetric with respect to the first bisector, is it correct to say

• $m-n=1$
• $m-n=-5$
• $m-n=-11$
• $m-n=-13$
• $m-n=15$

I think by first bisector, they mean the perpendicular bisector. So, the mid point is $\left(\dfrac{5m+2n}2,\dfrac12\right)$.

Not able to establish symmetry from here on.

Answer 1

@Jean Marie gave the hint that the first bisector meant the line $y=x$.

So, $$3m+n=-1\\2m+n=2\\\implies m=-3, n=8$$

So, the answer is $m-n=-11$