Help! I don't understand this

Question

Solve the system of equations

15x+31y = -3

x=−y+3​

Answer 1

15x +31y = -3

x = -y +3

substitute 2nd equation's x value into the 1st equation

15(-y+3) + 31y =-3

simplify and solve for y

-15y +45 +31y =-3

16y =-3-45= -48

y =-48/16 = -3

y =-3

x =-(-3) +3 =3+3 =6

x =6

solution = (x,y) = (6,-3)

check the answer, plug the x and y values into each of the original equations

x=-y+3

6 =-(-3)+3 = 6

15x+31y =-3

15(6) +31(-3)

=90-93 =-3