Answer :
Topic : Midpoint between points
- desk = (-5,2)
- box = (8, -8)
- door = (x, y)
formula:
[tex]\sf \bold{(x_m, y_m) = (\dfrac{x_1+x_2}{2}, \dfrac{y_1+y_2}{2})}[/tex]
=====================================
[tex]\hookrightarrow \sf \dfrac{x+(-5)}{2} ,\dfrac{y+2}{2} = (8, -8)[/tex]
separate taking coefficients:
[tex]\rightarrow \sf \dfrac{x-5}{2} =8, \ \dfrac{y+2}{2} = -8[/tex]
[tex]\rightarrow \sf x-5 =16, \ y+2 = -16[/tex]
[tex]\rightarrow \sf x =16+5, \ y= -16-2[/tex]
[tex]\rightarrow \sf x =21, \ y= -18[/tex]
coordinates of door: (21, -18)

Let that be (x,y)
So
[tex]\\ \rm\rightarrowtail (8,-8)=(\dfrac{x-5}{2},\dfrac{y+2}{2})[/tex]
[tex]\\ \rm\rightarrowtail \cfrac{x-5}{2}=8\implies x-5=16\implies x=21[/tex]
[tex]\\ \rm\rightarrowtail \cfrac{y+2}{2}=-8\implies y+2=-16\implies y=-18[/tex]
- (x,y)=(21,-18)