you always must choose one direction as plus and one as minus
since the normal force is perpendicular to the ramp direction any forces we have along the ramp direction we can use a different positive and negative direction
for example up from the ramp could be positive .. for the normal
but down the ramp direction could be positive for that because the two directions are never mixed
generally though you would pick the directions like a typical grid x positive to the right and y positive up and down but its not required
Fnet = sum of the forces
Fnet = Framp - Ffric
so this time we have
Fnet = mgsin A - Ffric
Ffric = mu*|n|
n = same magnitude as r
so the | | means length
|n| = mg cos A
|r| = mg cos A
Ffric = mu * mgcosA
Fnet = mgsin A - Ffric
Fnet = mgsin A - mu* mgcosA
Fnet = mg*(sinA - mu*cosA)
mu = coeficient of friction
thats how you do it
if the block was on a ramp that is changing direction like a hill then at the bottom it would start moving up
we picked down as plus but this would be more correctly stated as right along that axis so when it started going up the ramp that would be up is positive
so the
Fnet = -Framp - Ffriction
you see now that both forces are pulling back on the block
Fnet = -(Framp + Ffriction)
so instead of subtracting we add the forces
you can see when going down a ramp with no external forces except gravity the forces will subtract usually
and when going up the ramp with just gravity and friction as the external forces will make the forces add ..
but you have to WATCH the sign of the forces so that you dont get confused