2x+5y=21 (1st)
x+2y=6 (2nd)
Multiply (2nd) equation by 2 to eliminate x
2x+4y = 12 (2nd)
Now you have:
2x + 5y =21 (1st)
2x + 4y = 12 (2nd)
--------------------------------subtract
y = 9
Substitute y = 9 into (2nd) equation x+2y=6
x + 2(9) = 6
x + 18 = 6
x = -12
Answer
The value of x is -12