How Do You Maximize the Volume of a Prism with an Equilateral Triangle Base?

[rachael]

3 The cross-section of a solid prism is an equilateral triangle, The sum of the lengths of the edges of the prsim is 18 cm.
a. find the exact side length of the triangle in cm so that the volume is maximised.
b. Find the maximum volume in cm^3, correct to 2dp

i can't seem to find the correct answer for this question
could anyone please help me?
thank you

 

[pavadrin]

3 The cross-section of a solid prism is an equilateral triangle, The sum of the lengths of the edges of the prsim is 18 cm.
a. find the exact side length of the triangle in cm so that the volume is maximised.
b. Find the maximum volume in cm^3, correct to 2dp

i can't seem to find the correct answer for this question
could anyone please help me?
thank you

wheres ur working?