complex numbers

yes i think i figured that part know, but about ob=oa+oc doesnt that mean modulus b=modulusa+modulusc but that gives an answer with roots not the 3-4i that i am looking for

The other question you were asking about is complex mod-arg form which includes complex number divsiion. I can't see how you can have covered that, but not complex number addition/subtraction. To cover addition/subtraction for this question (if you really have not done it) is maybe ~1/2 hr. You posted the same question ~3 weeks ago and got similar advice about combining the two complex numbers a and b to get the solution. So as per #12, you know OA and OB and
OB = OA + AB
So for the desired side of the parallelgram
OC = AB = ...

You mentioned not covering vectors (complex numbers), the only thing I assumed about adding and subtracting vectors (tip to tail) is what is covered at gcse.

okay i think i have to mention that i am actually self studying the fp1 after finishing my gcses and looking into some exam questions for some topics that seam a bit harder before school start,so if you could explain things a little bit more, having said that why did you swap oc to ab i know they are equal but i want to find c and also i mentioned finding c with the ob=oa+oc and the modulus you didnt say if i could do that to find c so could i??

Complex addition and subtraction is part of FP1 and I don't know where the questions come from, but I'd bet they're based on the fact that youve covered it (do you have the textbook?). So have a read of that section (complex addition and subtraction) and simply rearrange the previous equation
OB = OA + AB
to get
AB = ...
and do the complex operation (addition or subraction). Its using rectangle form (not mod-arg) so re+i*im so the same form as a and b are given in the question. Upload what you try, it should be more than 1 (at most 2) simple lines.

Cant see much value in treating it as a linear equation question, where one of the objectives of complex numbers is to think about how geometry can be done simply using complex numbers/operators.

what do you mean where there coming from, yes i do have the edexcel textbook and i have been following that.
but isnt ob and oa the modulus the distance between the origin and the complex number, so can i just say that ab=-4i+3 isnt that the modulus, also where does swapping oc with ab help?