Relating outputs of parent function to inputs of nested function

Hi, I am going to try and explain myself simply in the hopes of getting a simple answer.

Let's say I have a function 'calculate' that takes the inputs [t,k,r,x] and outputs [A,B,C,D] as follows:

function [A,B,C,D] = calculate(t,k,r,x)

Now lets say I have another function that takes these outputs as the inputs, and spits out more, different outputs, eg.

function [M,N] = again(A,B,C,D)

How do I link [M,N] to say k and t? The overall aim is to minimise both M and N by optimising k and t, and I can guess that it has something to do with nested functions and passing parameters but I'm not sure how to start, and the start is all I want. Thanks