Let "s" be the son's present age.
Let "f" be the father's present age.

When the son will be as old as the father is now? - In f-s years.
At that time the son will be f years old and the father will be f + (f-s) = 2f-s years old.

Their ages added up at that time will be f + (2f-s) = 3f-s years.
It gives you the first equation 

3f - s = 126.    (1)


When the father was as old as the son is now? - In f-s years.
At that time the father's age was s years old and the son's age was s - (f-s) = 2s-f years old  (assuming that 2s-f >= 0).

Their ages added up at that time were s + (2s-f) = 3s-f years.
It gives you the second equation 

3s - f = 38.     (2)

Now express s from (1): s = 3f - 126,  and substitute it into (2). You will get

3(3f-126) - f = 38,   or

9f - 378 - f = 38,

8f = 38 + 378 = 416.

Hence, f =  = 52.

Father's current age is 52 years.

Then the sun's age is s = 3f - 126 = 30.   (notice that the intermediate assumption 2s - f >= 0 is valid !)