The equation would be (x+4)+x=18. X stands for the amount of pears Wilma bought, and (x+4) is the amount of pears Tania bought (because she bought four more pounds than Wilma).
(X+4)+x=18
2x+4=18 (now you subtract four from both sides)
2x=14 (to get x by itself, divide both sides by 2)
X=7
Wilma bought 7 pounds of pears :3