Rough Inclined Plane - Find Coefficient of friction

Homework Statement

a block of mass 2kg is in at rest on a rough plane inclined at 30°, it is in limiting equilibrium with with a force of 20N applied horizontally (to the ground).

Find the coefficient of static friction between the block and the plane.

The Attempt at a Solution

$R=mgcos30+20sin30\\ .\\ mgsin30-μR=20cos30\\ mgsin30-μmgcos30+μ20sin30=20cos30\\ 9.8-16.97μ+10μ=17.32\\ 9.8-26.97μ=17.32\\ -26.97μ=7.52\\ μ=-0.28\\$
Now I now my answer cannot be minus something as 0<μ<1, however, I am unsure as to whether it is my understanding of the question or the algebra I have got wrong. I am sure its a silly mistake, it usually is with me but any help or advice is greatly appreciated.

Mentor
$R=mgcos30+20sin30\\ mgsin30-μR=20cos30\\ mgsin30-μmgcos30+μ20sin30=20cos30\\$
When you substituted the two terms of R, only one of them got the minus sign.

Mentor
It looks like that was just a typo on your part and that your calculation did carry the minus sign throughout.

But you made an assumption about the direction of the friction force that may not be justified. Try interpreting the problem like so: If that 20 N force were any greater, the block would start to slide up the incline.