Wait, I think I figured it out:

• Let's divde the shape into a Triangle on the left, and a rectangle on the right.
• The triangle will have a hypotenuse of 5 and a base of 4 (5 - 1)
• We find the last side of the triangle by the Pythagoras' theorem: A^2 = B^2 + C^2 --> 5^2 = 4^2 + 3^2 -> The last side length is 3
• We get the left side of the rectangle by subtracting the triangle's side: 3-1= 2

The answer is 2!!