hello
to solve this problem, we need use law of indicies
[tex]\frac{a^m}{a^n}=a^{m-n}[/tex]
now we having know this, we can proceed to solve our problem here
[tex]\frac{16b^{42}}{4b^{25}}[/tex]
step 1, let's seperate them and use the expression above to solve it
[tex]\begin{gathered} \frac{16b^{42}}{4b^{25}}=\frac{16}{4}b^{42-25} \\ \frac{16}{4}b^{17} \end{gathered}[/tex]
step 2
divide 16 by 4
[tex]\begin{gathered} \frac{16}{4}b^{17} \\ 4b^{17} \end{gathered}[/tex]
