Answer: 27x+10
Step-by-step explanation:
Let both the boys are putting 'x' index cards in each envelope.
then, The number of index cards Marty put=[tex]15x+4[/tex]
And The number of index cards Stewart put=[tex]12x+6[/tex]
Now, the total number of index cards they have started with =[tex]15x+4+12x+6[/tex]
Hence, the total number of index cards they have started with =27x+10