19.5 in difference 
difference indicates that you'll be subtracting so 29 3/4 - 10 1/4 = 19.5
        
             
        
        
        
I think the surface area would be 148.31mm squared
See the attachment below
Thanks.
 
        
        
        
Try 9cm but I’m not quite sure please reply if it’s wrong or right
        
                    
             
        
        
        
The common ratio of the given geometric sequence is the number that is multiplied to the first term in order to get the second term. Consequently, this is also the number multiplied to the second term to get the third term. This cycle goes on and on until a certain term is acquired. In this item, the common ratio r is,
    r = t⁵/t⁸ = t²/t⁵
The answer, r = t⁻³.
The next three terms are,
    n₄ = (t²)(t⁻³) = t⁻¹
    n₅ = (t⁻¹)(t⁻³) = t⁻⁴
    n₆ = (t⁻⁴)(t⁻³) = t⁻⁷
The answers for the next three terms are as reflected above as n₄, n₅, and n₆, respectively. 
        
                    
             
        
        
        
OK im answering ............................................................................................................