Did I do something wrong ?

Answer: 0
Work Shown:
[tex]16 - 4^2 + 4 - 2^2\\\\16 - 16 + 4 - 4\\\\0 + 0\\\\0\\\\[/tex]
In the second step, the [tex]4^2[/tex] turns into 16 because [tex]4^2 =4*4 = 16[/tex]. Squaring a number means you multiply it with itself. So that's also why [tex]2^2 = 2*2 = 4[/tex]
Think of a 4 by 4 square with area 16; same goes for a 2 by 2 square of area 4.