This is like property of a parallelogram you have to do is basically follow its properties which is opposite sides are congruent
so we solve like this
subtract -3 on each side one is canceled since its equal to zero
and other is -3-5= -8
now subtract -3 on each side
-3x-x is -4 so,
-4x=8
divide its equal to 2
X=2
Hope it helps! have an awesome day you should probably study the properties of parallelogram