Answer:
-5<x<9
Explanation:
Within a triangle, the greater an angle, the greater the length of opposite side. So, since we know that 42° gives an opposite side of 23, and (3x+15)° gives an opposite side of 21, (3x+15) must be less than 42:
3x+15<42
3x<27
x<9
We also know that the angle (3x+15)° cannot equal zero or less than zero. So find x when 3x+15=0, and note that our value for x must be greater than this:
3x+15=0
3x=-15
x=-5
so, -5<x
Put both parts together to get: -5<x<9