Step-by-step explanation:
The perimeter is equal to the sum of all the sides. Since a triangle has 3 sides and you are only missing one,
P=10+12+x
We have the perimeter, so we can substitute P for 29
29=10+12+x
now we solve for x, the missing side
29=22+x
x=7
The length of the 3rd side is 7 [insert unit here].
