We need a normal vector for the plane the three given points define.
Let A(5,3,4)
B(-1,0,2) and
Γ(0,2,-1)
Then vector AB = < -6,-3,-2 > and BΓ = < 1,2 -3> and their cross product
AB x BΓ = <13,-20,-9>
Then the equation of the plane is 13x -20(y-2) -9(z+1) =0
13x -20y -9z = -31