Question

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

Answer 1

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