Question 960113
If I say:
{{{ z = a + b*x }}}
Then, 
{{{ b*x = z - a }}}
{{{ x = ( z-a )/b }}}
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{{{ x / ( a + b*x ) = ( z-a ) / ( b*z ) }}}
Doing the division, I get:
{{{ 1/b - a/( b*z) }}}
{{{ 1/b - ( a/b )*( 1/z ) }}}
I'll say:
{{{ w = 1/z }}}
then {{{ y = -(a/b)*w + 1/b }}}
with the substitution {{{ w = 1/( a + b*x ) }}}
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This may not be what you want -another opinion
is needed,  I think