x^2=9sin^2(3t), y^2=9cos^2(3t)=9-9sin^2(3t)=9-x^2 so y^2+x^2=9, which is the equation of a circle, radius 3, centre at the origin. This equation has eliminated the parameter t, so the original equations are not required to be solved, but rather t is a sort of go-between to introduce y to x and x to y, and to establish a relationship between them so that a graph can be plotted involving just x and y.