Because the sum of the lengths of any two sides must be greater than the third side we can say...
15+x>23
x>8
We can also say if x was the longest side...
15+23>x
x<38 from these to inequalities we can see that:
8<x<38
and the other two sides equal 38
So the range of possible perimeters is
46<p<76 or if you prefer...
p=(46,76)