I am better at substitution, but you want to get the equations as similar as possible, but are the opposite, so you can cancel it out.
the given figure is a rhombus, so it's area =

where, d1 and d2 are diagonals of the rhombus,
and since, the diagnonals of rhombus bisects each other, the values of their diagonals are
(2 × x) and (2 × 15) =》2x and 30 respectively,
so, by finding the area using the given diagonals and equating it with the real area, we get :
=》

=》

=》

=》

so,
value of x = 10 units
Answer:
(C) 10m + 7n - 14
Step-by-step explanation:
[Q-R]+[S-T] =[(7m+3n)-(11-2m)]+[(n+5)-(-m-3n+8)]=
=[7m+3n-11+2m] + [n+5+m+3n-8]=[9m+3n-11] + [4n+m - 3]=
=9m+3n-11+4n+m-3=10m + 7n - 14