(i screenshot the whole thing. pls pls d: )

This is a little bit weird, but it's not hard if you just
do it one step at a time.

Here's the important part that you need to know
in order to solve this problem:
 
            All four sides of a square are equal.

Got that ?   Good. 
Here we go.

-- First, take the left and right sides.
   They must be equal, so

                                         2y - 5  =  y - 1

Add  5  to each side:             2y   =  y + 4

Subtract 'y' from each side:      y  =  4 .  

The left side is          y-1  = 3

The right side is      2y-5  =  2(4) - 5
                                        =   8 - 5
                                        =    3 . 

Good !  We have the left and right sides equal.
Do we really need to do the other two sides ?
They told us it's a square, so can't we just say
         the top and bottom are also 3 ?
Well, maybe.  But maybe they're trying to trick us,
         and the top and bottom aren't really 3 .  I think
         we ought to check it out.  It's more good math practice.

-- Take the bottom side ... it's  3y-9 .

But you know what 'y' is !
We just calculated that  y=4,
so
                               3y - 9  =  3(4) - 9  =  12 - 9  =  3 .

Great !  The bottom side is  3 .