We want to calculate the following
[tex]2.31\cdot10^{-6}+5.87\cdot10^{-4}[/tex]Using the properties of exponents, we have that
[tex]10^{-6}=10^{-4}\cdot10^{-2}[/tex]So we have
[tex]2.31\cdot10^{-6}+5.87\cdot10^{-4}=2.31\cdot10^{-2}\cdot10^{-4}+5.87\cdot10^{-4}[/tex]So, if factor 10^-4 on the right side, we have
[tex]10^{-4}(2.31\cdot10^{-2}+5.87)[/tex]Note that
[tex]2.31\cdot10^{-2}=0.0231[/tex]Then,
[tex]5.87+2.31\cdot10^{-2}=5.87+0.0231=5.8931^{}[/tex]So we have that
[tex]2.31\cdot10^{-6}+5.87\cdot10^{-4}=5.8931\cdot10^{-4}[/tex]