can someone help me with this?

how can i find the projection point (Q') of the point Q = (1,2,1) on the plan that passes through the points A = (1,0,0) ; B = (1, 1, 0) and C = (2,1,1)?

i found the normal vector of the plan, which is n = (1,0,-1)

so the plan equation is x - z -1 = 0, right? but now i dont know how to find Q', pls help :(