Final answer:
The expression (4 x 10⁴) (9 x 10⁻⁴) simplifies to 3.6 x 10¹.
Step-by-step explanation:
The expression (4 x 104) (9 x 10-4) can be simplified by multiplying the whole numbers and adding the exponents. So, (4 x 9) (10⁴ x 10⁻⁴) = 36 x 104-4 = 36 x 100 = 36.
In scientific notation, 36 is written as 3.6 x 101.
Therefore, the expression is simplified into 3.6 x 101.