Answer:
Let x = the third side
In a triangle, the sum of any 2 sides must be larger than the third side.
I believe this is called the triangle inequality theorem.
We can construct 3 inequalities to obtain the range of values for the third side.
(Inequality #1) 12 + 4 > x
16 > x
(Inequality#2) 12 + x > 4
x > -8 (we can discard this ... we know all sides will be positive)
(Inequality #3) 4 + x > 12
x > 8
So when we combine these together,
8 < x < 16
X (the third side) must be a number between 8 and 16. but not including 8 and 16
