Question 148379
Start with
{{{(5/a)-(4/y)=1/b}}}


Subtract 5/a from both sides and get 
{{{-(4/y)=(1/b)-(5/a)}}}


Multiply everything by y

{{{-4=(y/b)-(5y/a)}}}


Multiply everything by y again
{{{-4y=(y^2/b)-(5y^2/a)}}}


Divide everything by -4
{{{y=((y^2/b)-(5y^2/a))/-4}}}