The answer provided in the NCERT solutions of Class VIII Maths , Chapter - Factorisation Ex - 14.3 Question 5) vii) is not given completely.Why???

 here is the complete solution for that question :

39y³ ( 50y² - 98) / 26y² ( 5y + 7)

= 13 x 3 x y² x y x 2(25y² - 49) / 13 x 2 x y² (5y + 7)

= 3y [(5y²) - (7)²] / (5y + 7)

= 3y ( 5y - 7) (5y + 7) / (5y + 7)

= 3y(5y - 7)

hope this helps