Answer  
Common difference = -4.5 
Explanation.  
An arithmetic sequence is a sequence in which the next term is found by adding a constant to the current term. This constant is called the common difference. 
If a, b, c, d, e, ….is an arithmetic sequence, then the common difference will be, 
Common difference = b-a=c-b=d-c=e-d….. 
Now the question above common difference is; 
Common difference = (-22.5)-(-14)=-4.5