Complex sentence: Further development with the 20,500-lb (9300-kg) thrust Lyulka AL-7F1, resulted in the I-7K that first flew in January 1959 and proved capable of flight at Mach 2.35.
Split into simpler sentences: Further development with the 20,500-lb (9300-kg) thrust Lyulka AL-7F1, resulted in the I-7K. The I-7K first flew in January 1959 and was capable of flight at Mach 2.35.