How convert the binary number 110101001 to decimal then to octal?

You can use the Windows calculator to do the conversions. If you want to learn how to do it yourself:

To convert binary to decimal, multiply the right-most digit with 1, the second digit (from the right) with 2, the third with 4, etc.

To convert to octal, group the bits from the right to the left, in groups of 3. Convert each group to a decimal digit.