13/39 + (х + 25/39) = 1

1 = 39/39, пишем 39/39 вместо 1.

13/39 + (х + 25/39) = 39/39

х + 25/39 = 39/39 - 13/39

х + 25/39 = 26/39

х = 26/39 - 25/39

х = 1/39