Basic Algebra

If the mean of $$ p, q, r $$ is $$ M $$ and $$ pq + qr + rp = 0 $$, then the mean of $$ p^2, q^2, r^2 $$ is: ...

If $$x$$ and $$y$$ vary inversely to each other, and $$x = 7$$ when $$y = 12$$, find $$y$$ when $$x = 21$$....

If $$ \cos(6\theta) = \sin(3\theta) $$, then the value of $$ \theta $$ is: ...