Reckon, compute (9)
I believe the answer is:
calculate
I believe this is a double definition.
'reckon' is the first definition.
(synonyms)
'compute' is the second definition.
(I know that compute can be written as calculate)
(Other definitions for calculate that I've seen before include "'Work out, compute (9)'" , "Compute or reckon" , "Work out mathematically" , "Solve by mathematical procedure" , "Think about" .)