Help! I don't know what i'm doing wrong.

Can we see your work and how you tried to solve it?

Draw  WY...this will be the hypotenuse of  isosceles right triangle WYX

Its length will be   √2  * leg length  = √2 * 4    =   √32  units

Angle WYX  =  45....so angle  WYZ  =  m [angle ZYX]  - m [ WYX]  =  135 - 45  = 90°

So triangle  WYZ is a right triangle  with WZ  the hypotenuse  and legs  WY  and YZ 

And we can use the Pythagorean Theorem to find  YZ  =   "a"

So

WY^2  + YZ^2  =  WZ^2

(√32)^2  + YZ^2  =  9^2

32  +  YZ^2  =  81

YZ^2  =  81 - 32

YZ^2  =  49       take the square root of both sides

YZ  =  "a"   =   7

-2+(-3)-5+2(3)