Let the coordinate of $D$ be $(x, y)$. In a parallelogram, mid point of diagonal AC coincides with the mid-point of diagonal BD. Mid point of $AC= \left( \\;\frac{3+3}{2}, \\;\frac{6+2}{2} \right) =(3,4)$Mid point of $BD= \left( \\;\frac{x+5}{2}, \\;\frac{y+10}{2} \right)$$\because$ Mid point of $A C=$ Mid point of $B D$ $(3,4)= \left( \\;\frac{x+5}{2}, \\;\frac{y+10}{2} \right)$Equating, $\\;3=\\;\frac{x+5}{2} \\;\text{ and }\\; 4=\\;\frac{y+10}{2}$ $\\;x=1 \\;\\; \\;\text{ and }\\; y=-2$ Coordinates of the fourth vertex $D(1,-2)$