Question 4323
First, assign the variables.
Let H = Huilan's age
T = Thomas's age. 

H = T + 6
H + T = 72    Subsitute H from the first equation into the second equation.

(T + 6) + T = 72   Solve for T.
2T + 6 = 72  Subtract 6 from both sides.
2T = 66       Divide both sides by 2.
T = 33   Thomas's age is 33 years.
H = T + 6 = 33 + 6 = 39  Huilan's age is 39 years.

Check:  
T+6 = 33+6 = 39
T+H = 33+39 = 72