Layla's Problem Solving

I am learning to draw a picture to help me solve a problem.

The problem that I had to solve was:

Larenzo has 3 times as many marbles than Janet does. Janet has 7 marbles. How many marbles does Larenzo have?

Miss Hughes came into our class. We wrote on our whiteboards. I wrote three squares and seven tally marks in the squares. I got the answer. It was 21. I found it hard.