Angle issue - Need help with finding coordinates

I have worked with trig before, but kinda just on overload and can’t figure this out.

Imagine two movieclips, both on the same Y axis. I want one MC to be able to diagonally move to the other MC X axis, all while making a perfect diagonal line until it reaches 100y.

So just like this picture, that MC moves in a perfect diagonal until it matches the other MC.x, and 100y (no matter what, it can’t go past 100y but must equal 100y)

I probably made it sound more complicated than it is. Anyway, any help would be hugely appreciated.

Thanks.