Answer:
3
Step-by-step explanation:
It should be 3 because when you look at it divide them all and each one gives you 3. If it's not then sorry.
This is a word problem signifying an algebraic expression.
Translating this to mathematics:

add 4 to both sides.
divide 16 to both sides.
this is:
The answer would be 21 because its the same length as the other side.