Answer:
2a +2b +4
Step-by-step explanation:
The perimeter is the sum of the lengths of the three sides. If x represents the length of the third side, then ...
(3a +8) +(5a +b) +x = (10a +3b +12)
Subtracting from the equation everything on the left side that is not x, we get ...
x = (10a +3b +12) -(3a +8) -(5a +b)
= a(10 -3 -5) +b(3 -1) +(12 -8)
= 2a +2b +4
The length of the third side is 2a +2b +4.
