According to the properties of parallelograms the diagonals bisect each other. What this means is that each "half" of a diagonal is equal to each other. With this in mind you can set each half equal to each other in an equation like so...
3x + 3 = 6x - 57
Now you must combine like terms
60 = 3x
Isolate x by dividing 3 to both sides
60/3 = 3x/3
20 = x
Hope this helped!
~Just a girl in love with Shawn Mendes