Hi,

to rearrange for in the first equation, you can substract then multiply by . So you find :

So :

Setting to make it a bit simpler, we are left with :

So :

Substituting back the value we chose for , we get :

Finding the value of is now straightforward

Does it make sense ? Remember not to get stopped by impressive square roots and stuff : as long as there are no terms in them, they can be considered a constant (and thus substituted to some letter, in my example)