If function date problem

Hello,
i am trying to use excel to give me two values depepnding if its a monday or a wednesday

i have a date box so every week a person will put the current date in the box
they will only use the spreed sheet on a monday or a wednesday so i only need the funtion to work for those two days
when it is a monday i want it to give them a set figure minus a figure they enter
if it a wednesday i want it to give them a different figure

examples below

If(b3=monday,[F3-C3],[G3-C3])
B3 = date cell
F3 = set figure for monday
C3 = entered figure
G3 = set number for thursday

so if F3 = 4 and its monday and they enter 2 into cell C3 then the figure it gives them should be 2 so it totals 4 and if it thursday it should be 4 so it totals 6

really hope im making sense

thanks to any one who can help