Click an item in the list or group of pictures at the bottom of the problem and, holding the button down, drag it into the correct position in the answer box. Release your mouse button when the item is place. If you change your mind, drag the item to the trashcan. Click the trashcan to clear all your answers. Perform the operation indicated, then place the answer in the proper location on the grid.

The perimeter of a triangle is 7a - 11b. If two of its sides are 2a + b and a - 9b, what is the third side?