Klowie'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?

We were doing some maths with Miss Hughes. We did some problem solving. We learned some bonds and counting in tens. I drew three boxes with seven dots in each one. I got the answer it was 21. I found it tricky.