Remember that an exponent means repeated multiplication.
The exponent of 4 means you have four [tex]\big(b^2\big)[/tex]'s mutiplied together:
[tex]\big(b^2\big)\cdot\big(b^2\big)\cdot\big(b^2\big)\cdot\big(b^2\big)[/tex]
And then each exponent of 2 inside means you have two [tex]b[/tex]'s multiplied inside each set of parentheses.:
[tex]\big(b\cdot b\big)\cdot\big(b\cdot b\big)\cdot\big(b\cdot b\big)\cdot\big(b\cdot b\big)[/tex]
There are 8 b's there when you expand this out.
