iterate

I'm not quite understanding the problem. It sounds like a "solve a system of equations" kind of problem, but I can't be sure of what is given and what our your unkowns are. It sounds like you are given B10, C10, C18, and D12; and you need to solve for x and y.

It is also not clear to me, when you say you want to "iterate" exactly what algorithm you are trying to use. Are you allowed to use Solver for this problem? Solver is usually how we implement numerical algorithms in Excel, and I expect it would make short work of this problem. If you are not allowed to use Solver, it might help if we knew what you were supposed to learning, or what skill this exercise was supposed to demonstrate.

I find that these kind of problems often need to be handled outside of Excel until the problem is in a form that we can understand how to program it. This may mean algebraically manipulating it into a "root finding" problem. Or rearranging until you have equations where y=f(x) and x=g(y). We can then make an educated guess for one of them, then use successive approximations until it converges on a solution. Or whatever makes sense for this problem.