Let $\vec{a}=a_1 \hat{i}+a_2 \hat{j}+a_3 \hat{k} \quad a_i>0, i=1,2,3$ be a vector which makes equal angles with the coordinates axes OX, OY and OZ . Also, let the projection of $\overrightarrow{\mathrm{a}}$ on the vector $3 \hat{\mathrm{i}}+4 \hat{\mathrm{j}}$ be 7 . Let $\overrightarrow{\mathrm{b}}$ be a vector obtained by rotating $\overrightarrow{\mathrm{a}}$ with $90^{\circ}$. If $\vec{a}, \vec{b}$ and $x$-axis are coplanar, then projection of a vector $\vec{b}$ on $3 \hat{i}+4 \hat{j}$ is equal to