Question 191164
3(x - y)= 15 
2x + y = 7


2x + y  = 7
y = 7 - 2x 
Substituting this value for y in eq. (i) gives us: 
3{x-(7-2x)} = 15
3(x – 7 + 2x) = 15
3x – 21 + 6x = 15
9x  =  36
x  =  4

Substitute this value for x in eq. (ii) 2x + y = 7. This give us:
2(4) + y = 7
y = - 1

You made an error when substituting the value of y (7 – 2x) into the 1st equation (see above). As seen, either equation can be substituted in order to find the values of x and y. So, the answer is (4, - 1).