What is the error in: -6/7 + -1/2 = -12/14 + -7/14 = -19/28 and how do you fix it?

Sherrie is baking a pie for her family. she leaves the room and comes back to 35% of the pie having been eaten before she can put the topping on. how much 1 square inch strips of dough will she need for the top, now that a portion is missing? round your answer to the nearest while nimber.