Question 515073
Cat = 2 lb
Dog = 14 lb

Let both gain x lb 
(Dog + x)=3*(Cat+x)
14+x=3(2+x)
14+x=6+3x
x-3x=6-14
-2x=-8
-2x/-2=-8/-2
x=4

Cat = 2lb + 4lb = 6 lbs
Dog = 14lb + 4lb = 18 lbs

The cat wil weigh 6 pounds.