You used Pythagorean theroem which is correct as this does form a right triangle however 50 is the hypotenuse as the ladder is propped up against the wall. 30 would then be the height of the triangle and the base is unknown so...
50 squared minus thirty squared gives 1600 so:
Therefore x is the square root of 1600 which is 40
Draw a diagram. You will see that 50 is the hypotenuse and that the height is 30, and from your scratch work, I can see that you know the Pythagorean Theorem, so I will do the calculations: We have that 30^2 + x^2 = 50^2, or 900 + x^2 = 2500, or x^2 = 1600 so x = 40.
