datagridview cells multiplying

Question

Hi 

I have one problem with datagridview. I wanna to multiplying two cellse and displya result in third cell?

example: 

A   |  B  |  C

10.50 | 2  | 21,00

5.20  | 4 |  20,80

7,3  | 5  | 36,50

after that i wanna to sum C column and display that in textBox.

Column A is type decimal

Column B is integer

Column C should be decimal too

how to do that and how to display decimal value in textbox?

I have found couple solutions on internet but they don't work for me.

ty