Stephanie adds pennies, nickels and quarters to a scale until the mass of the combined coins is $75$75​ grams. Each penny has a mass of $2.5$2.5​ grams, each nickel has a mass of $5$5​ grams and each quarter has a mass of $5.7$5.7​ grams. Create an equation to model this situation, where $x$x​ is the number of pennies, $y$y​ is the number of nickels and $z$z​ is the number of quarters that Stephanie can put on the scale so that the mass of the combined coins is exactly $75$75​ grams.