Choose the correct simplification of the expression b5 ⋅ b4. b b9 b20 b−1

When multiplying like terms, you add the exponents:

b^5 · b^4 = b^(5+4) = b^9

FYI - when dividing like terms, subtract the exponents.

The only time we multiply exponents is when they distribute across parentheses:
(x³)² = x^6