The equation of a plane sinusoidal wave propagating in the direction given by the unit vector $\mathbf{\hat k}$ is
$$\mathbf E = \mathbf E_0 \cos\left(\frac{2\pi}{\lambda}\mathbf{\hat k} \cdot \mathbf r -\frac{2\pi}{T} t+\phi_0\right)$$
Here, $\mathbf{\hat k}$ is the unit vector at right angles to the wavefront, whose distance from the origin – as you'll see if you draw a diagram – is $\mathbf{\hat k} \cdot \mathbf r$.
Writing, for convenience, $\mathbf k=\frac{2\pi}{\lambda} \mathbf{\hat k}$ and $\omega=\frac{2\pi}{T}$,
$$\mathbf E = \mathbf E_0 \cos(\mathbf k \cdot \mathbf r -\omega t+\phi_0)$$
Expanding the dot product:
$$\mathbf E = \mathbf E_0 \cos(k_xx+k_yy+k_zz -\omega t+\phi_0)$$
Comparing with the wave you are considering, we see that

$$k_x=a, k_y=0, k_z=b$$
So the components of $\mathbf{\hat k}$ are
$$\frac {a\lambda}{2\pi}, 0, \frac {b\lambda}{2\pi}$$
The propagation direction therefore lies in the x-z plane, at an angle $\arctan{\tfrac ba}$ to the x axis.
 
We note that, since $\mathbf{\hat k}$ is a unit vector,
$$\left(\frac {a\lambda}{2\pi}\right)^2+ 0^2+\left( \frac {b\lambda}{2\pi}\right)^2=1.$$