The value of b is -6.
Explanation:
The expression is 
To determine the value of b, we shall solve the expression.
Applying exponent rule,
, we get,

Applying exponent rule,
, we have,

The expression is of the form,
then 
Applying this rule, we get,

Dividing both sides by 4, we have,

Hence, the value of b is -6.