C= number of acres of corn
W= number of acres of wheat
total is 180 acres
c+w=180
want to plant 3 times as many corn as wheat
c=3w
substitute 3w for c in other
ctw=180
3w+w=180
4w=180
divide both sides by 4
subtract
c=3w
c=3(45)
c=135
135 acres of corn and 45 acres of wheat