Enter the value of p so the expression 5/6 - 1/3n equivalent to p(5-2n)

You said                          5/6 - 1/3 n  =  p (5 - 2n)

Divide each side by (5 - 2n) :   

                                   p = (5/6 - 1/3 n) / (5 - 2n) .

That's about all you can do with it.
As you can see, the required value for 'p'
depends on the value of 'n'.

If you want to get rid of those awkward fractions,
I guess you could multiply top and bottom by  6 .
Then you have ...

                           p = (5 - 2 n) / 6·(5 - 2n) .

Oh, my goodness !  Look at that !

                           p = 1/6 .

It doesn't even depend on 'n' at all.  It's always true,
no matter what 'n' is.

I am surprised.