Midpoint : M (3, 5 )
Endpoint : C (0,0)
Apply the midpoint formula:

For the x coordinate:
m1 = (x1+x2) /2
3 = (0 + x2 ) /2
Solve for x2
3 (2) = x2
6= x2
x2 = 6
For the y-coordinate:
m2 = (y1+y2) /2
5 = (0+y2) /2
Solve for y2
5(2)= (0+y2)
10 = y2
y2 =10
Coordinates of endpoint B: (6,10)