First, you should find the ratio of how much more sugar Maggie added. Set it up like this:
6x = 9
x = 9/6
x = 3/2
We now know that Maggie added 3/2 more sugar than she was supposed to. To keep consistent with recipe, we have to multiply this ratio with the amount the recipe calls for:
3/2(2) = 3
Maggie should add 3 tablespoons of mustard.