Write a system of equations to describe the situation below, solve using substitution, and fill in the blanks.
Sally, an artist, plans to paint and sell some miniature paintings. She just
bought some brushes for $72, and paint and canvas for each painting costs
$21; she will sell each painting for $57. Once Sally sells a certain number of
her paintings, she will be breaking even. How much will Sally have earned?
How many paintings will that be?