How to calculate x and y of any object inside an image when stretched?
Provided that the stretch is simply linear and consistent across the image, then the displacement of the point is proportional to the magnification factor.

For the X-Axis:


At $100px$ width, the point was at $30$
At $150px$ width, where would the point be? (Let's call this our unknown $x$).


$$x=\frac{150}{100} \times 30 = 45$$

Similarly, for the Y-Axis, the point will now be at $60$.

Hope this helps.