Overengineered solution to Stupid problem

#include <stdio.h>

// Structs definition
typedef struct Point {
  float x;
  float y;
} point;

typedef struct Line {
  float a;
  float c;
  float from;
  float to;
} line;         //f(x) = ax + c in [from, to]


// Global variables definition
line parts[3];


// Functions defintion
line solver(point, point);
float solution(float);



int main() {
  point Data[4] = {{0, 0}, {15, 20}, {27, 55}, {35, 100}};

  for (int i=0; i<3; i++) {
    parts[i] = solver(Data[i], Data[i+1]);
  }

  float x;

  scanf("%f", &x);
  printf("%f\n", solution(x));

  return 0;
}



// Creates each part of the function
line solver(point p, point q) {
  float a = (q.y - p.y)/(q.x - p.x);
  float c = p.y - a*p.x;

  line f = {
    .a = a,
    .c = c,
    .from = p.x,
    .to = p.y,
  };

  return f;
}


// This is The Function
float solution(float x) {
  for (int i=0; i<3; i++) {
    line p = parts[i];
    if (x>=p.from && x<=p.to) {
      return (p.a*x + p.c);
    }
  }
  return -1;
}