please help, I don't understand this

After substitution in the first equation :
7x + x +21 = -19
= 8x + 21 = -19
by subtracting 21 :
8x = -19 - 21
8x = -40
by dividing over 8 :
x = -5
after substitution in the second equation :
2y = -5 + 21
2y = 16
by dividing over 2 :
y = 8
pls give me brainliest