Question 547227
{{{3qr+9t=7u}}}
We need to isolate r.   
One trick I learned is do your BEDMAS backwards.  So starting with S/A, we see that we have +9t we must get rid of.  
{{{3qr+9t-9t=7u-9t}}}
{{{3qr=7u-9t}}}

Okay, now there is no more addition or subtraction on the same side of the equation as the r.  the next step is looking for any division or multiplication.  We have a 3 and a q multiplied with the r, so we must divide these out.  
{{{3qr/3q=(7u-9t)/3q}}}
{{{r=(7u-9t)/3q}}}

And there it is!