If your parametric equations are

    x = 3cos(t)

    y = 3sin(t),


then SQUARE each equation and then add them.


You will get


    x^2 + y^2 = 9cos^(t) + 9sin^2(t) = 9*(sin^2(t) + cos^2(t)) = 9.


So, you just converted the given parametric equations to rectangular form
and this converted (resulting) equation is


    x^2 + y^2 = 9.


It is the standard equation of the circle  of the radius of 3 units centered at the origin of the coordinate system.


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