Hey there, you must use static balance equation in x and y Direction and also calculate moment around point b and equal the Sum 0 ,as the result reaction at point a will be calculated, now you can use static equations in both X and Y directions to find and reactions in point b.

Also use the fact that reaction force at point A is normal to the slant surface, i.e. RA=RA{sin 20o; cos 20o}.

Victor J. Аdlucky has provided the solution you solve in respect to vertical and horizontal axis but don't forget that A is roller so you only resolve it vertically Kanticha Korsesthakarn