Assign values to registers and add and subtract them

Question

I am completely lost with this. I need to program that calculates the following expression, using registers: varA = (varA + varB) − (varC + varD), where varA, varB, etc., are v

Answer 1

mov    eax,varA
add    eax,varB
mov    ecx,varC
add    ecx,varD

This all makes perfect sense, eax = varA + varB. ecx = varC + varD.

At this point, just

sub eax, ecx

to get eax = (varA + varB) - (varC + varD).

I'm not sure what was intended with

mov  ebx,varA
sub  edx,varD

That sets ebx = varA, and then sets edx = edx - varD. edx wasn't defined before, so this is a random number. Or maybe edx was originally cleared to zero, in which case edx is now equal to -varD. I wouldn't count on it though.