Subtracting the left and right sides of x+y=−9 from the corresponding sides of x+2y=−25 gives (x+2y)−(x+y)=−25−(−9), which is equivalent to y = −16. Substituting −16 for y in x+y=−9 givesx+(−16)=−9, which is equivalent to x=−9−(−16)=7.