Question 949608

Basic qustion here, say I have: 

(16.5)²/33² + (12)²/b² = 1 
144/b² = 1 - (272.25/1089) = 816.75/1089. 
b² = 144 / (816.75/1089) = 192 

my question is what happened between 144/b² to b² I thought if you have a problem like a fraction to get the 144 out I would think I'd have to multiply it out rather it looks like something else happened, could you explain what happened here? 

Thanks
<pre>Let's pick it up from here:
144/b² = 1 - (272.25/1089) = 816.75/1089. 
{{{144/b^2 = 816.75/1089}}}
{{{144/b^2 = .75}}}
{{{.75b^2 = 144}}} --------- Cross-multiplying 
{{{b^2 = 144/.75}}}
{{{b = sqrt(144/.75)}}}, or b = 192