Completing Zohaib's answer, I would suggest
D1 - =IF(C1="Yes",A1+B1,A1)
assuming that if we have no "YES" then we just want to copy A1 over.
Also, if you want to keep the current file data, with the first value in column A, then the "YES" in column B, then
in F1 type "5"
in the Formulas menu / Define name set the name "Tax" for cell F1
in C1 enter
C1 - =IF(B1="Yes",A1+Tax,A1)