x+y =27 sum of two numbers is 27
x=y+3 One of the numbers is three more than the other
substitute x=y+3 into the first equation
(y+3) + y = 27
combine like terms
2y + 3 = 27
subtract 3 from each side
2y = 24
divide by 2
y =12
x = y+3
x = 12+3
x =15
the two numbers are 12 and 15