Since it’s indicated that BC and AD are congruent, you can set the two sides/equations equal to one another:
3x+3=x+21
Then, you can solve for x:
3x+3=x+21
2x=18
x=9
Plugging in 9 into either equation of BC or AD should give you 30 for both sides:
3(9)+3=30
(9)+21=30
For CD, you can also plug in 9 for x:
2(9)-9=18
Since AB and CD are labeled as congruent, both sides are 18.
x=9
AB=18
BC=30
CD=18
AD=30
