Question 363859
Let g = weight of Grok last year in kg 
    m = weight of Grok's mother last year in kg

    m = 3g

  m-4 = 2(g+9)
 
since m = 3g
 (3g)-4 = 2(g+9)
  3g -4 = 2g + 18
  3g-2g = 18+4
      g = 22
 
      m = 3*22
        = 66
  
Note that: 
   g+9 = weight of Grok now
   m-4 = weight of Grok's mother now

Grok weighs 31 Kg now
Grok's mother weighs 62 Kg now