Solution:
Total number of students=273
Let x students choose Tiger. Then we can write
Number of students who choose Bulldog 
Number of students who choose Lion 
So we can write

Hence a total of 39*2=78 student choose Tiger.
I attached a picture, see if you can see what I'm doing. Basically, I start by squaring everything to get rid of the radical, but just remember at the end to always, always check your answers, because squaring your answers could get rid of some negative numbers that could result in imaginary answers. The answer I came up with was a=5,-2