There were 72 different ways Gabby could have chosen her car.
There were 4 choices of engines
There were 3 choices of transmission
Let the number of choices for color were n
Now she could chose 1 engine out of 4 choices in 4 different ways.
She could chose 1 transmission out of 3 choices in 3 different ways.
She could chose 1 color out of n choices in n ways.
So 4 × 3 × n = 72
12n = 72
Dividing by 12 we get
n= 6
So there were 6 choices of color to be chosen from.