I understand that
Now I have this problem, find the missing a,b or c length.
So I say that
or am I doing it wrong?
If not then I get confused on this next question.
but how can in this question?
The square root of 8 is not 8. The square root of 8 is 2 times the square root of two.
You seem to be reading "a = square root of 8" as "a squared = 8".
In you "a = " statements, try squaring both sides. Maybe that'll make it clearer.