To find the angle that this equation forms with the x-axis, we must first find the slope.
Slope is -3/5. To find the angle of this slope consider the definitions of tangent:
tan(θ) = opp/adj = sin(θ)/cos(θ)
Tangent is the vertical component divided by the horizontal component, aka rise over run, aka the slope. So we can say:
or θ≈-.5404 radians