another substitution question

[ginny1029]

x - 2y = 7
3x - 21 = 6y

first equ x - 2y = 7
x = 2y + 7

so substitute in second equation

3x - 21 = 6y
3(2y + 7) - 21 = 6y
6y + 21 - 21 = 6y
6y = 6y
6y/6 = 6y/6
y = y

so now i dont know how to substitute back into the first equation
or do i consider it an infinite number of solutions

 

[Loren]

Notice that the two original equations are really the same equation, one in disguise.

(1) x - 2y = 7
(2) 3x - 21 = 6y

(2) 3x - 6y = 21
(2) 3(x - 2y) = 3(7)
(2) x - 2y = 7

Therefore, you originally had one equation in two unknowns written down twice, which, as you say, has an infinite number of solutions.