x + 8 = rachel's age in 8 years {added 8 onto current age}
3x + 8 = mom's age in 8 years {added 8 onto current age}
x + 8 = 0.5(3x + 8) {in 8 years rachel will be equal to half her mom}
x + 8 = 1.5x + 4 {used distributive property}
-0.5x + 8 = 4 {subtracted 1.5x from each side}
-0.5x = -4 {subtracted 8 from each side}
x = 8 {divided each side by -0.5}