Verified answer

The surface z = 18 cos(xy) is the level surface for the value 0 of the function f(x,y,z) = z - 18 cos(xy).

Gradient f = (18y sin(xy), 18x sin(xy), 1) , Gradient f(2, π/6, 9) = ((3^1.5)π/2, 6(3^1.5), 1).

The tangent plane is given by (3^1.5)π/2(x-2) + 6(3^1.5)(y-π/6) + (z-9) =0