Answer:
20
Step-by-step explanation:
In a parallelogram diagonals bisect each other.
[tex] \therefore \: 6x - 57 = 3x + 3 \\ \\ \therefore \: 6x - 3x = 57 + 3 \\ \\ \therefore \: 3x = 60 \\ \\ \therefore \: x = \frac{60}{30} \\ \\ \therefore \: x = 20[/tex]
