Please explain...................

substitute x = 6 - y into 7x = -3y
7x = -3y
7(6 - y) = -3y
42 - 7y = -3y
42 = 4y
y = 10.5
x = 6 – 10.5 = 6 – 10.5 = -4.5
Now you know x = -4.5 and y = 10.5 you can find the value of C
c = 4x + 2y
c = 4(-4.5) + 2(10.5)
c = -18 + 21
c = 3