You'd set the equation up as such: 3+M +M = 71 (I got 71 because the age increased by 4 years which means it increased 4 more than 67). You'd combine like terms - 2M +3 = 71, subtract both sides by 3 thus equally 2M=68, then divide both sides by 2, equally M = 34. Plug Michelle's age in at 34 and then Shelly is 3 years older at 37.