I have some questions about 'LOOP'

I am a korean so I can't write well about my questions. But I will try to write.

I want to express 'Loop'.

Let's we suppose two equations. One is X=Y^2+2Y+3. The last is Y=X^2+3X+5.

I can find adequate value by finding convergence between X and Y.

But I can't express in MATHCAD.

I think I need to use 'While' to express..

How can I express it to use 'while'?