Change javascript code in function node

[chandra123]

@bakman2
Hi,
I have written one rule as
sensorData=JSON.parse(msg.payload);
var a=sensordata.dp1;
var a=sensordata.dp2;
var c=a+b;
msg.payload=c;
return msg;

this is addition rule of 2 datapoints
Now I want to change it to multiplication rule by external rest-api or anything possible.
how can I do that?