Question 160536
{{{21000=12000(1+r/4)^36}}}
{{{21/12=(1+r/4)^36}}}
Take the 36th root of both sides(when we do this we'll have both positive and negative solution).
We'll add that at the end.
{{{(21/12)^(1/36)=(1+r/4)}}}

{{{(21/12)^(1/36)-1=r/4}}}
{{{r=4((21/12)^(1/36)-1)}}}and
{{{r=-4((21/12)^(1/36)-1)}}}and
or approximately r=0.0627, -0.0627
.
.
.
Just in case, your equation looks like this,
{{{21000=12000((1+r)/4)^36}}}
{{{21/12=((1+r)/4)^36}}}
{{{(21/12)^(1/36)=(1+r)/4}}}
{{{4(21/12)^(1/36)=1+r}}}
{{{r=4((21/12)^(1/36))-1}}}
Then approximately, 
r=3.063,-3.063