given vertices

O (0,0,0)

P (1,2,-2)

Q (0,2,-1)

R (1,0,1)

PQ (-1,0,1)

Coord xyz of Q minus coord xyz of P

PR (0,-2,3)

Coord xyz of R minus coord xyz of P

Edit: and then calc the cross product of pq x pr before applying the formula below

Then apply 1/2 |square root a^2 + b^2 + c^2| = square root of 17 / 2