We have the segment BC, M is the midpoint
M (4,5)=(xm,ym)
B(0.9) =(x1,y1)
C(x,y)=(x2,y2)
We have the next formula

xm=4
x1=0
x2=x
we substitute the values

we isolate x

the for the coordinate y

we have
ym=5
y1=9
y2=y
we substitute the values

then we isolate the y

the coordinate of C is (8,1)