The distance of point I(3, 4,5) from the yz-plane is 

When we draw a perpendicular line from P(3,4, 5) on the yz-plane the r-coordinate will be zero and the other
coordinate y and e will be 4 and 5, i,.e. coordinate on yz-plane be Q(0, 4,5).
 Distance between P and Q = PQ

$\begin{array}{c}=\sqrt{(3-0{)}^2+(4-4{)}^2+(5-5{)}^2}\\ =\sqrt{3^2+0^2+0^2}=3\text{ units }\end{array}$