Remark
You really do not have to figure out the side lengths of the purple or light blue squares. The area of those squares are given.
The short answer is 85 units, and before reading on, you should see if you can get that answer.
Solution
The solution to this depends on the Pythagorean Theorem.
The Pythagorean Theorem says that a^2 + b^2 = c^2
Notice that all three terms describe areas of squares. The smallest square has an area of a^2 or 35 units^2. The middle square is 50 units^2
So the largest square must be 35 + 50 = 85 units