Answer:
5x+y=21......equation 1
x-3y=9......equation 2
from1:y=21-5x.....equation 3
substitute y by 21-5x in equation 2
x-3(21-5x)=9
x-63+15x=9
x+15x=9+63
16x=72(we divide both sides by 16)
16÷16=72÷16
therefore x=9/2 or x=4,5
substitute x by 4,5 or 9/2 in equation 3
y=21-5(9/2) or y=21-5(4,5)
y=-3/2 or y=-3/2
prove
5(9/2)-3/2=21
-3/2-3(9/2)=9
