system of equations

x - 3y = 1     →    x = 1 + 3y      (1)

3x - 5y= 11     (2)

Sub ( 1)  into (2)

3 ( 1 + 3y ) - 5y= 11

3 + 9y  - 5y = 11

3 + 4y  = 11       subtract 3 from both sides

4y  = 8               divide by 4

y = 2

And

x = 1 + 3y 

x = 1 +  3(2)  =  1 + 6  = 7