First, express in slope-intercept form:
y=mx+b
Where:
m= slope
b= y-intercept
isolate y:
5x+3y= 0
3y= -5x
y= -5/3 x
m= -5/3
Apply the formula:
m= tan Θ
Where Θ is the inclination
-5/3 = tan Θ
arc tan -5/3 = Θ
Θ = -59°
since m < 0, add 180°
180 - 59
Θ= 121°
