Step 1: The projection of a vector \( \mathbf{v} = 3\hat{i} + 4\hat{j} + 5\hat{k} \) on the xy-plane is given by the vector \( \mathbf{v}_{xy} = 3\hat{i} + 4\hat{j} \). Step 2: The length of the projection is the magnitude of \( \mathbf{v}_{xy} \):
\[
\left| \mathbf{v}_{xy} \right| = \sqrt{3^2 + 4^2} = 5.
\]