float t = 0;

void setup() {
  Serial.begin(9600);
}

void loop() {
  t = t + 0.1;
  Serial.println( linear_fun(t));
  delay(2);
}

int linear_fun(float x) {
  return  -5 * (x - 6) * (x - 6);
//note: here, you can put math problems here (like linear equaation) for it to solve.
}