I did not read it the moment I saw your first line.

The gradient is not a real number! It is a vector. For example given then (where means partial derivative along and so on ... ). Notice this is not a number, it is a vector.

Here are the steps.

1)Given .

2)Find .

3)Evaluate this gradient at the specific point you are given.

4)The vector in #3 is orthogonal to the surface. Therefore, it is the normal to the tangent plane to the surface at this point. Since you know the normal vector and the point you can describe the tangent plane.