Let A (0, 0, 0), B (3, 4, 0) and C (3, 4, 6)
Area, (Δ)=(Δx2+Δy2+Δ2)
Now, Δx=21∣∣y1y2y3z1z2z3111∣∣=21∣∣044006111∣∣ =21[1(24)]=12
Similarly, Δy=21∣∣006033111∣∣=21[1(−8)]=−9
and Δz=21∣∣033044111∣∣=21[12−12]=0
Area of Δ=122+(−9)2+(0)2 =144+81=225=15 square units