To answer that question we need to apply equations of movement ( from Newton´s laws )
In equilibrium:
∑F = 0 or ∑Fx = 0 ; ∑Fy = 0
Solution is:
a) F(sf) = 251 [N]
b) Fn = 691 [N]
From the attached drawings we can see: ( Body free diagram)
∑ Fₓ = F(sf) - Pₓ = 0 where P = m×g = 735 [N] ( the weigth)
and Pₓ = P× cos20°
Then F(sf) = Pₓ × sin 20°
F(sf) = m×g×cos20° = 735× 0.34202 [N]
F(sf) = 251.3847
F(sf) = 251 [N]
rounding to the nearest number
F(sf) = 691 [N]
∑ Fy = 0
∑ Fy = Fn - Py = 0 Py = P×cos20° Py = m×g×cos20°
Py = 735×0.939693 [N] Py = [N]
Fn = Py = 690.674 [N]
rounding to the nearest whole number
Fn = 691 [N]