how do to validate a inputted function

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C

can i have a simple sample program that lets the user input a function then evaluates the function then gives the output value.
the values of the variables are pre-defined.

lets say i have this code that accepts int values from the user

#include<stdio.h>
main
{
int A,B,X,VAL;
printf("enter a number:");
scanf("%d",&A);

printf("enter a number:");
scanf("%d",&B);

printf("enter a number:");
scanf("%d",&X);

Val= (A * X) + B;

printf("the value of the function is %d",Val);
}

now I need a really simple code in which the user this time will be the one to input the function. my knowledge on C is that wide. I've never saw a program that asks the user to input a function instead a value so this the first time I've encountered this sort of problem.

Im just going to use the sample code on understanding how a program like that is done so I can proceed with the machine problem our teacher tasked us to do.


thank you :)))