When using substitution, you want to solve an equation for one variable that doesn’t make the equation worse to work with.
The general idea is that you want to find a variable that has a coefficient of 1 or -1.
In 2.5x - 7y = 7.5, both variables have “bad” coefficients.
In 6.2x - y = 1, the y has a coefficient of -1. That’s where you want to start.