How do I solve system of equations by subsitution

You need to solve for x or y in one equation and and substitute it into the other equation in place of the variable that corresponds to it.

Do the math. she originally had 50 dollars on her gift card and she had 12 dollars left on her gift card after she finished her shopping spree. if you subtract 50 minus 12 you get thirty eight dollars as your answer. so therefore, ciara spent 38 dollars total o