Answer:
-5 < x < 9
Step-by-step explanation:
By Hinge theorem,
"If two sides of a triangle are congruent to the two sides of another triangle, and the included angle of one triangle is larger than the included angle of the second, then the 3rd side of triangle one will be larger than the third side of the second"
23 > 21
Therefore, 42° > (3x + 15)°
42° - 15° > 3x
27 > 3x
9 > x
And (3x + 15) > 0
3x > -15
x > -5
Therefore, range for x will be,
-5 < x < 9
