multiply several different rows in a colum beginner

see attached file for example. so i have a chart. say it has 5 colums a,b,c,d,e and 3 rows 1,2,3. (it really has hunreds) all rows have different values (numbers). some rows need to be multiplied. lets say b1 is 3 and b2 is 5, and d colums are blank. what if i need to autommatically multiply each value in b colum by 2 to make d1 now have 6 and d2 to have 10? is there a way to do this without having to conditionally format each row hundreds of times?