Ineed with this . i have a test on that. find the unknown length

Ok, from your picture, it looks like you have a square and a right triangle next to each other

one of the definition of a square is that all 4 sides are equal, and its given to you that the square has a side length of 3
since the triangle shares a side with the square, that means that shared side has a length of 3

ok, now how about determining the unknown length x

well the base has a length of 5, but you also know that when you add the length of the square side plus x, you should get 5

3+x = 5
and you can solve for x  (subtract 3 from both sides)
x=2

now how about determining side length y
well since you know this is a right triangle, you can apply pythagorean theorem

where C is the hypothenuse of the triangle, or the longest side

when you substitute the known values (3 and x)
you get
3^2 + x^2 = y^2

since you already know x=2
3^2 +2^2 = y^2

and you can solve for y

any questions?