Door is 77 inches high, 35.25 inches wide and 1,75 inches thick how much paint is needed

If you wanted to paint the entire door (even the top and bottom which people don't usually paint)

the surface area is

SA=2 (LW+LH+WH)

where L=length, W=width and H=height

given

L=35.25

W=1.75

H=77

SA=2 (35.25*1.75+35.25*77+1.75*77)

SA=2 (61.6875+2714.25+134.75)

SA=2 (2910.6875)

SA=5821.375

you need enough paint to cover 5821.375 square inches of door