How to subtract and add polynomials

We want to add and subtract the polynomial

having the same powers.

This is a tricky question that Reina and Sam are multiplying (84)5 (73)9and their work are (84)5 (73)9 = 84 + 573 + 9 = 89712    Reina's(84)5 (73)9 = 84⋅573⋅9 = 820727 Sam'sActually they are doing or multiplying using exponents.
Reina’s work (8⁴)₅(7³)⁹=8⁴⁺⁵+ 7³⁺⁹= 8⁹ 7¹²This is incorrect  because she added exponents instead of multiplying exponents
Sams work (8⁴)⁵ (7³)⁹=8⁴ˣ⁵ 7³ˣ⁹= 8²⁰ 7²⁷This is correct  because Sam multiplied exponents correctly
Exponents are multiplied not added.