Mom's age 3 years ago: 39-3 = 36

Xiaohong's age 3 years ago: 13-3 = 10.

Three years ago, my mother was Xiaohong: 36 ÷ 10 = 3.6.

(The "Xiaohong this year 13 years old" in your question seems to be "next year", so it can meet the math requirements of Grade Two. )