For example, if Y = X*2
X is in cell A1
Y is in cell A2
we usually write "= A1*2" in A2 " to find out the Y value when we know X
or we could write "= A2/2" in A1 to calculate X when we have Y
but these functions work one direction only
what if I sometimes know X and some other times know Y

How do I write a two-way function where I can put in known X value or Y
value to calculate for the other unknown value (Y or X)