The point at which a material ceases to be elastic and becomes permanently plastic, the point at which it yields, is called its yield point. The magnitude of stress at which this transition occurs is known as the material’s yield stress or strength. The yield strength is a material constant that represents the limit of its elastic behavior.
As it is a material constant we need to know the material. But also as yield strength is stress than:
where Area is a product of width and thickness: A = 4*300 = 1200 m2; Force -- is the force that deals on the metal sheet in the yielding moment.