Flattening a circle

A circle of radius $5$ is centered at the origin. We want to "flatten" the circle in the $x$ -direction so that it becomes an ellipse having the same perimeter as the circle, and such that its semi-major axis length is $a = 7$ . Find the semi-minor axis length $b$ and enter $\lfloor 1000 b \rfloor$ as the answer.