Answer :
[tex]Greatest\ common\ factor\ is\ the\ highest\ number\ by\ witch\ 45, \ 51\ and\\ 63\ can\ be\ divided\\\\
45:3\\
15:3\\
5:5\\\\
51:3\\
17:17\\\\63:3\\21:3\\7:7\\\\GCF=3\\\\Greatest\ common\ factor\ of\ 45,\ 63\ and\ 51\ is\ equal\ to\ 3.
[/tex]