finding quadratic equation, when given 3 points

hi. i have the answers to these questions, although i really can't seem to work them out. i know you should sub them into the equation y=ax^2+bx+c but when i have the 3 equations i dont know how to solve them simultaneously. can someone please help?! :)

Find the equation of the quadratic which passes through the points with coordinates:

a

(−2, −1), (1, 2), (3, −16)

so far i have:

-1=(-2)^2a - 2b + c

2= a + b + c

-16= 9a + 3b + c

but how do i go further?

thankyou in advance :)