Using triangle inequality theorem, we can say 'sum of any two sides of a triangle is greater than the third side'.
So we can write,
10 + 18 > x
That is, x < 28
let assume a,b and, c as three sides of a triangle.
then a + b > c, also b - a < c
The difference between any two sides should be less than the other side.
So, 18 - 10 < x
x > 8
Hence the range of x will be,
8 < x < 28