Here's a way in Word 2007; later versions should work similarly. This is for the new-style equation editor, which I believe is what you're using since your screenshot is in the Cambria Math font.
in your equation, after the equals sign, insert left bracket from "Cases and Stacks":
Put the cursor just right of the two placeholder boxes:
Hit Backspace twice to replace the two boxes with a single box:
Insert a matrix with three rows and two columns:
The result will be:
Fill in the contents of your matrix:
Right-click the left column, select Column Alignment, and pick Left:
Right-click the right column, select Column Alignment, and pick Right: