Answer:
The missing expression is 'b'.
Explanation:
We have been given an incomplete sentence. We are supposed to fill in the given blank.
Sentence:
The value of the _____ determines whether the graph of an exponential function increases or decreases from left to right.
We know that an exponential function is in form 
, where, 
a = Initial value,
b = For growth 
 and for decay 
.
Therefore, the value of 'b' determines whether the graph of an exponential function increases or decreases from left to right.