diff --git a/rtchallenge/src/matrices.rs b/rtchallenge/src/matrices.rs
index bb03631..6363d13 100644
--- a/rtchallenge/src/matrices.rs
+++ b/rtchallenge/src/matrices.rs
@@ -1,5 +1,5 @@
 use std::fmt;
-use std::ops::{Index, IndexMut, Mul};
+use std::ops::{Index, IndexMut, Mul, Sub};
 
 use crate::{tuples::Tuple, EPSILON};
 
@@ -161,7 +161,6 @@ impl PartialEq for Matrix3x3 {
     }
 }
 
-#[derive(Copy, Clone, Default)]
 /// Matrix4x4 represents a 4x4 matrix in row-major form. So, element `m[i][j]` corresponds to m<sub>i,j</sub>
 /// where `i` is the row number and `j` is the column number.
 ///
@@ -194,6 +193,7 @@ impl PartialEq for Matrix3x3 {
 /// let t = c * b * a;
 /// assert_eq!(t * p, Tuple::point(15., 0., 7.));
 /// ```
+#[derive(Copy, Clone, Default)]
 pub struct Matrix4x4 {
     m: [[f32; 4]; 4],
 }
@@ -764,7 +764,7 @@ impl fmt::Debug for Matrix4x4 {
         if f.alternate() {
             write!(
                 f,
-                "{:?}\n  {:?}\n  {:?}\n  {:?}",
+                "\n  {:8.5?}\n  {:8.5?}\n  {:8.5?}\n  {:8.5?}",
                 self.m[0], self.m[1], self.m[2], self.m[3]
             )
         } else {
@@ -838,6 +838,21 @@ impl Mul<Tuple> for Matrix4x4 {
     }
 }
 
+impl Sub for Matrix4x4 {
+    type Output = Matrix4x4;
+
+    fn sub(self, m2: Matrix4x4) -> Matrix4x4 {
+        let m1 = self;
+        let mut r: Matrix4x4 = Default::default();
+        for i in 0..4 {
+            for j in 0..4 {
+                r.m[i][j] = m1.m[i][j] - m2.m[i][j];
+            }
+        }
+        r
+    }
+}
+
 impl PartialEq for Matrix4x4 {
     fn eq(&self, rhs: &Matrix4x4) -> bool {
         let l = self.m;