pls help me with this simple step
i dont know how that last step came,
From what you have
$\displaystyle
y - a \sin ^3 t = - \tan t \left(x - a \cos ^3 t\right)
$
$\displaystyle
y \cos t - a \sin^3t \cos t = - x \sin t + a \cos^3t \sin t
$
$\displaystyle
y \cos t = - x \sin t +a \sin^3t \cos t + a \cos^3t \sin t
$
$\displaystyle
y \cos t = - x \sin t +a \sin t \cos t\left( \underbrace{\cos^2t + \sin^2t}_{=1}\right)
$
so
$\displaystyle
y \cos t = - x \sin t +a \sin t \cos t$.