Find the x-intercept of parabola with vertex (1,-17) and y-intercept (0,16)?

Y=a(x-1)^2 -17 is the equation of the parabola.
at (0,16), 16=a(0-1)^2 -17.  Solve for a = 33.
Equation is then y=33(x-1)^2-17 so when y=0,
(x-1)^2 = 17/33
x-1 = + and - square root of 17/33
x = 1.7177 and 0,2823 as the intercepts

This is a known bug in earlier versions of the game.

Explanation:

An easy way to solve this that I have seen work before is to completely destroy the 'new' portal that appears in the overworld. Once done, you should be able to enter your portal and exit from the same portal.

To help avoid this, you should also never install a portal in your world in an area that is at all enclosed. The portal system seems to dislike portals appearing in places they should 'not be' based on the initial construction of the world.

Also avoid dying, as it can cause random nether portals to form in parts of your world.