\tan \alpha = k \
\tan \alpha = -0,4 \
\Rightarrow \alpha \approx -21,8^{\circ}

f: (2 * x) + (5 * y) - 10 = 0
A = Point(f)
B = Point(xAxis)
C = Intersect(f, xAxis)
α = Angle(A, C, B)
k = Slope(f)