Answer:
a. 13
Step-by-step explanation:
x - youngest sister
x + 1 - middle sister
x + 1 + 1 - oldest sister
x + x + 1 + x + 1 + 1 = 39
3x + 3 = 39
3x = 36
x = 12
x+ 1 = 12 + 1 = 13
