{q,r}=QRDecomposition[{{12, -51, 4}, {6, 167, -68}, {-4, 24, -41}}];
q//MatrixForm

-> 6/7 3/7 -(2/7)
-69/175 158/175 6/35
-58/175 6/175 -33/35

r//MatrixForm
-> 14 21 -14
   0  175 -70
   0  0  35