I have been given two questions, both of which i ahve no clue on what to do

linear transformation T : R^2-->R^2 with standard matrix M=[a,0;0,b] a>b>0

a) show the circle x^2+y^2=r^2 is transformed by T to the ellipse:

(x^2)/(a^2)+(y^2)/(b^2)=r^2

hint, let P(rcos(theta), rsin(theta)) be a point on the circle and show that P is transformed to Q(arcos(theta), brsin(theta)), a point on the ellipse.

b)derive a formula for the area of an ellipse