Find the length of the third side of the triangle if its perimeter is 12 cm and the length of the other sides is 3 cm and 4 cm.

Brief notation of the problem condition:
Triangle;
P∆ = 12 cm;
a = 3 cm;
b = 4 cm;
c =?
Decision:
The perimeter of a triangle is equal to the sum of all three of its sides. As a formula, the perimeter of a triangle can be written as:
P∆ = a + b + c;
c = P∆- (a + b);
Substituting the known values of the perimeter and side lengths, we get:
c = 12- (3 + 4) = 12-7 = 5 (see)
Answer: 5 cm.