Question 302020
x-2y=7 ... equation 1
3x-21=6y ... equation 2

Let's x = 1 

x-2y = 7 
1 - 2y = 7 
-2y = 7 -1 
-2y = 6 
y = -3

Put y = -3 and x = 1 into equation 2

3x - 21 = 6y 
3x - 6y = 21
3(1) - 6(-3) = 21
3 -(-18) = 21
3 + 18 = 21
21 = 21 

Therefore, x = 1 and y = -3