Answer is 20 centimeters
Step-by-step explanation:
Triangle ABC is equilateral, because AB=BC=AC=a. Then
m∠A=m∠B=m∠C=60°.
Let point D lie on the ray BC to the right from points B and C and let CD=x. Then BD=a+x, AB=a.
Consider triangle ACD. In this triangle, m∠ACD=180°-m∠ACB=180°-60°=120°.
By the cosine theorem,

Since
then

and

Therefore, you get double inequality
or 
Since 2 cases = $48
÷ 48 by 2 to get the cost of one case of paper
48÷2=$24 per case
and each case has 12 pack of paper
so $24 for 12 packs of paper
180÷12=15, which gives you the number of case you need
15 cases × $24= $360 for 180 packs