ince successfully solved the system of equations −5x y = − 21 and y = x − 1. Which of these statements could correctly describe what he did? He substituted x − 1 for y in the equation −5x y = − 21 and solved for x. He then substituted the value of x back into one of the equations and solved for y, getting x = 4 and y = 5. He substituted x − 1 for y in the equation −5x y = − 21 and solved for x. He then substituted the value of x back into one of the equations and solved for y, getting x = 5 and y = 4. He substituted x − 1 for y in the equation −5x y = − 21 and solved for y. He then substituted the value of y back into one of the equations and solved for x, getting x = 4 and y = 5. He substituted x − 1 for y in the equation −5x y = − 21 and solved for y. He then substituted the value of y back into one of the equations and solved for x, getting x = 5 and y = 4.