Explanation:
to be a triangle the sum of any 2 sides must be larger than the third side.
so,
x + 10 > 18
x + 18 > 10
18 + 10 > x
the first inequality gives us
x > 8
the third inequality gives us
28 > x
the second inequality would give us
x > -8
but with the first inequality x > 8 we have already a stronger criteria than x > -8 (if x > 8 is true, then so is automatically x > -8, but not the other way around).
therefore x > 8 is the narrower criteria and answer.
so, we have in combination
8 < x < 28