Breaking it down:
"return" tells the script to exit the function so everything after this line does not get executed.
return (((D1*2.036+D3)/(273+D5))-((D2*2.036+D4)/(273+D6)))*238*(3050/D7)
You are missing the closing squigglies
D1.onkeyup=function() {calc(D1,D2,D3,D4,D4,D6,D7)}...