/sci/ - Science & Math

Guys, how do I solve this.
<span class="math">a+b+c = 18[/spoiler]
<span class="math">a^2+b^2+c^2 = 756[/spoiler]
<span class="math">a^2=bc[/spoiler]

I couldn't get past:
<span class="math">a^2+b^2+c^2 = 42(a+b+c)[/spoiler]
Just to let you know, one can easily find the answers using wolfram, but I want to figure out how to do it without a computer.