The lawnmower accelerates in the positive horizontal direction, so that the net horizontal force is, by Newton's second law,
(70 N) cos(-50°) = m (1.8 m/s²)
where m is the mass of the lawnmower. Solve for m :
m = ((70 N) cos(-50°)) / (1.8 m/s²)
m ≈ 25 kg
The lawnmower presumably doesn't get lifted off the ground, so that the net vertical force is 0. If n is the magnitude of the normal force, then by Newton's second law,
n - m g + (70 N) sin(-50°) = 0
n = m g + (70 N) sin(50°)
n = (25 kg) (9.8 m/s²) + (70 N) sin(50°)
n ≈ 300 N
