How do I solve these by either substitution/ elimination? I'd also like to know how to solve them using matrices.

$\displaystyle U_n = an^{2}+bn+c=0$

$\displaystyle U_1=4$

$\displaystyle U_2=10$

$\displaystyle U_3=18$

I need to find the values of the constants a, b, c are.