A good way to check if something is even is if you plug in 1, then -1 and see if they are equal...
First one)
1^4-1^2=0
(-1)^4-(-1)^2=0
Thus f(x) is even.
Second one)
1^2-3(1)+2=0
(-1)^2-3(-1)+2=
Thus f(x) isn't even
Third one)
Let's use 2 for this
sqrt(2-2)=0
sqrt(-2-2)=does not exist
So it's not even
Fourth one)
abs(1)=1
abs(-1)=1
So it's even