Answer: 12a^3
Explanation: You would get the missing side to complete the perimeter by using the pythagorean theorem so 3^2+4^2=c^2 so you would get 9+16=c^2 which would become 25=c^2. That would get you 5 as the square root of 25 is 5 so in total, it would all be 3a^3+4a^3+5a^3 which would equal 12a^3.
(^ are for exponents) I might be wrong but hopefully it helps a bit. :)
