Simple javascript, multiplying variables and displaying in alert box

Gawd… cant believe im asking this. I can understand and explan code in front of me just fine, but its writing it thats the pain!

I have a form with three text boxes.

One text box is for the user to insert a length value (numerical)

One text box is for the user to insert a height value (numerical)

One box is for the user to select a price (numerical)

When we process the form, I just need to display an alert box that multiplies the two values from the first 2 text boxes, adds 25% of that to the figure, and then multiplies that final figure by what was entered in the third text box.

So ie, in the alert box: “The total area is '(length)(width)25%+"and the cost is '((length)(width)25%)(cost)”

Totally off, but i think it helps illustrate what i need. Nothin too complex.

Dead easy really, but I just dont know the proper syntax!

Anyone able to shed some light for me?