How to find the value of a variable based on when another variable equals a certain number

So what I'm trying to accomplish is to find the value of X right before Y is greater than D in my function

function [Result] = hw(D)

My X equation and my Y equation are

 X = V.*T
 Y = 0.5.*(9.81).*(T.^2)

Now I have to find X at the instance right before Y is greater than D. I would then have to incorporate that X into a vector so I made a new X called X_2. To solve X_2, I'd have to find the time (T) at which Y is less than D. If I do something like

T_ = (sqrt(2.*(Y)/9.81))

that would give me the T_ so I can put T_ into X_2, but how would I make it so I find the T right before Y is greater than D?

Answer 1

[Walter Roberson]

No you cannot do that. Your X and your Y have no computation in common, so it is meaningless to talk about an X before or after a particular Y.

[lauuser1]

Sorry, I copied my coding wrong. X and Y should have T in common.