add & subtract values from userform to specific column in sheet

what i fill in multiple combobox on userform it supposes the same what is existed in the sheet to add or subtract
for instance the data in sheet for specific brand is AA 20-W50 208L Q8 THI 124 but if i fill on user form differently like this
AA 20-W50 208L cas THI 5 as you see in col 3 not match as what existed in the sheet so the code can't run correctly because not match
i hope this help