In order to prove the Pythagorean Theorem using the given diagram, the crucial step would be:
"The area of the purple and red rectangles on the left add to be the area of the yellow square."
This is because the Pythagorean Theorem states that in a right triangle, the square of the length of the hypotenuse is equal to the sum of the squares of the lengths of the other two sides. By showing that the area of the two rectangles on the left is equal to the area of the yellow square, it is possible to demonstrate the relationship between the sides of the right triangle and provide evidence for the Pythagorean Theorem.