we know that
Step 1
Find B
Applying the law of sines
a /sinA=b /sin B------> solve for sin B
sin B=(b/a)*sin A------> sin B=(15/12)*sin 23-----> sin B=0.4884
B=arc sin(0.4884)--------> B=29.2°
Step 2
Find C
we know that
A+B+C=180°
C=180-(23+29.2)-------> C=127.8°
Step 3
Find c
a /sinA=c /sin C------> solve for c
c=a*sin C/sin A-------> c=12*sin 127.8/sin 23------>c=24.3 units
the answer is
24.3