x = (1/2)y + 6
x + y = 21
Substitute (1/2)y + 6 as x into the equation x + y = 21 since x = (1/2)y + 6.
[(1/2)y + 6] + y = 21
0.5y + 6 + y = 21
1.5y = 15
y = 10
Same thing, now substitute 10 as y in the other equation.
x + y = 21
x + (10) = 21
x = 11