The correct option is B −2m2−10n+20
Perimeter of a triangle is a+b+c (side+base+side)
Here, Perimeter is 9m2−2n+8
and two sides are 4m2+3n , 7m2+5n−12
Let other side be x
Therefore,
perimeter=a+b+c
=>9m2−2n+8=4m2+3n+7m2+5n−12+x
=9m2−2n+8−4m2−3n−7m2−5n+12=x
=−2m2−10n+20=x
Therefore,
the other side is −2m2−10n+20