If 3x +5y + 7z = 49

9x + 8y + 21z = 126,

then find the value of 2y.

6

We have two equations and 3 variables 

So, put z = 0 (because z is extra)

3(3x + 5y = 49)   (Multiply by 3 for making 9x)

9x + 8y = 126

9x + 15y = 147  ...(i)

9x + 8y = 126   ...(ii)

Eq.(i) - Eq. (ii)

(9x + 15y) - (9x + 8y) = 147 - 126

9x + 15y - 9x + -8y = 147 - 126

7y = 21

y = 3

2y = 6