The pythagorean theorem states that if a triangle is a right triangle, then the sum of the squares of the legs of the triangle is equal to the square of the hypotenuse or a² + b² = c².
The converse of the pythagorean theorem on the other hand states that if the sum of the squares of 2 sides of a triangle is equal to the square of the third side or a² + b² = c², then the triangle is a right triangle.
The converse switches the order of the if and then statements.
On the whiteboard, I have also written it.
Image provided.