The value of b is -6.
Step-by-step explanation:
The expression is 

To determine the value of b, we shall solve the expression.
Applying exponent rule, 
 , we get,
, we get,

Applying exponent rule, 
 , we have,
, we have,

The expression is of the form, 
 then
 then 

Applying this rule, we get,

Dividing both sides by 4, we have,

Hence, the value of b is -6.