The Cartesian equation of the plane is ax + by + cz = d.

ai + bj + ck is a normal to the plane. If you know a normal, you know the values of a, b and c. To get the value of d, substitute the known point (1, -1, 3) into the Cartesian equation:

a(1) + b(-1) + c(3) = d.

Oh yes ...... the values of a, b and c are got from the normal. If the surface has the equation x = f(x, y), thenanormal is:

.

Substitute x = 1 and y = -1 to get a normal at (1, -1, 3).