To multiply \(0.75\) by \(2.8 \times 10^{-5}\), you do the multiplication of the two numbers first:
\[ 0.75 \times 2.8 = 2.1 \]
Now, include the power of ten:
\[ 2.1 \times 10^{-5} \]
So, the result of multiplying \(0.75\) by \(2.8 \times 10^{-5}\) is:
\[ 2.1 \times 10^{-5} \]