want to add dpp

[crtvrm]

hi,

in invoice, i want to add 'dpp' under sub total and the value off 'dpp' is :

dpp = subtotal * 0.92

I added in invoice template with this :  

{math equation="( x * y )" x=$subtotal y=0.92}

but the result is empty.

please help.

thanks in advance

[crtvrm]

i changed the code to this 

{assign var="mysubtotal" value=$subtotal|@}
{math assign="dpp" equation="x * y" x=$mysubtotal y=0.92}
{formatCurrency($dpp)}

and the result is white blank page.

please tell me what's wrong with the code above

thanks in advance