4. The age of Mr. Martin is \( 2 \) years older than \( 6 \) times of her son's old. \( 3 \) years ago the sum of their age is \( 31 \) years old. a) Determine and solve its system of equations b) Find their age when \( 8 \) years later.
a) Now Mr. Martin is 32 and her son is 5.
b) When 8 years later Mr. Martin is 40 and her son is 13.