To subtract, we have to put them both into same powers of 10.
Let's change first number to power of -8.
[tex]6.26\times10^{-4}=62600\times10^{-8}[/tex]Now we subtract the number and tag along a 10 to the power -8.
So,
[tex]\begin{gathered} 62600\times10^{-8}-12.52\times10^{-8} \\ =62587.48\times10^{-8} \end{gathered}[/tex]In proper scientific notation, it will be
[tex]6.258748\times10^{-4}[/tex]