Do the following how to add an equation in your document see Working with Microsoft Equation. There are two ways to insert matrix in Microsoft Word. Under Equation Tools on the Design tab in the Structures group click the Script button.
On the Equation Tools Design tab in the Symbols group in the list of symbols choose In the lower box of the subscript enter 0.
Drag it and drop it on any place on a toolbar. Under Equation Tools on the Design tab in the Structures group click the Script button. The Object dialog box appears. Then click the Equation button in the Symbols button group.