First group the terms as given below. \[ x^{3}+6 x^{2}-6 x-36=\left(x^{3}+\square\right)-(6 x+36) \]

To group the terms appropriately, we should look for patterns or common factors in the expression. The original expression is: \[ x^{3}+6 x^{2}-6 x-36. \] We can rewrite it as follows: \[ x^{3} + 6x^{2} + (-6x - 36). \] Now, let's fill in the square. Notice that \( 6x^2 \) can be considered as our second grouping and we can represent \( -6x - 36 \) as maintaining the signs properly. Thus, we have: \[ x^{3} + 6x^{2} + \square, \] where \( \square \) corresponds to the sum of the grouping \((-6x - 36)\). So, the filled grouping becomes: \[ x^{3} + 6x^{2} + \square = (x^{3} + \square) - (6x + 36). \] The term to represent would be \( 6x^2 \) with \( \square = 6x^2 \): \[ x^{3} + 6x^{2} + (-6x - 36). \] Thus, the result is: \[ x^{3}+\underline{6x^{2}}-(6x + 36). \]