Find a common line and use normals for the acute plane angle

For each pair of planes, find their line of intersection and acute angle. All parameters are real.

(a) $\mathbf r\cdot(1,-1,3)=3$ and $\mathbf r\cdot(2,-1,2)=5$.

(b) $\mathbf r=(1,2,7)+\lambda(1,1,0)+\mu(1,0,1)$ and $\mathbf r=(1,2,7)+s(1,2,4)+t(0,1,1)$.