Tell me how they get the answer for number 7

Jenny's cats -> J
Daenna's cats -> D
J = 3D
J + D = 12 => D = 12 - J
I can now replace D in the first equation
J = 3 * (12 - J)
J = 36 - 3J
J + 3J = 36
4J = 36 => J = 36/4 = 9 cats
9 + D = 12 => D = 12 - 9 = 3 cats
Jenny has 9 cats, Daenna has 3 cats
Answer
They combined the total number of cats they each had together.
Step-by-step explanation:
jenny had 3 times the number of cats as Daenna, so you do 3d then you do 3x3=9 ,then do 9+3=12 so the answer would be 9. this might be wrong