The formula for a is $\mathbf{a} = (\mathbf{v.u}/\|\mathbf{u}\|^2)\mathbf{u}$, where the dot denotes the scalar product and $\|\mathbf{u}\| = (4^2+(-4)^2+5^2)^{1/2}$ is the norm (or length) of u. Once you have found a, let b=v-a, and keep your fingers crossed that b is orthogonal (perpendicular) to u.