A very, VERY silly question

[crazylegs]

Evening all.

I appear to have lost all semblance of computer knowledge.

I am trying to create a formula in Excel where (for example) cell d10 is the result of cell d7 minus the values in cells d8 and d9.

Answers on a postcard please

(and please be gentle! )

[crazylegs]

It would appear my brain has once agani returned and I've worked it out...