check the picture below
[tex]\bf r^2=x^2+y^2\implies 2r\cfrac{dr}{dt}=2x\cfrac{dx}{dt}+2y\cfrac{dy}{dt}\implies \cfrac{dr}{dt}=\cfrac{x\frac{dx}{dt}+y\frac{dt}{dt}}{r}
\\\\\\
\cfrac{dr}{dt}=\cfrac{(5\cdot -0.5)+(3\cdot 0.4)}{\sqrt{34}}[/tex]
if it's a negative value, thus a negative rate, thus is decreasing, if it is a positive value, then increasing.
