How is the underlined step coming?

How is the underlined step coming? ( „ by the P(n) be defined as 24. , Which whenqeN ve that + J) is true whenever P(k) is true. Vish to "0 27+39-5=2.7'. T +3.5k. 51-5 51 + 3.5k • 5-5 = 7 [24q-3.5*+5J+ 15.5k -5 =7x24q-21.5k+35+ 15.5k-5 =7x24q-6.5k+30 -7x24q-6(5k-5) have * h evefote. toe fox Pto a multiple =7x24q-24p -24k r, is natural number. 24. Thus P(k+ l) is true who N k) istrue tical