Recall that a plane given by the equation has the vector (a,b,c) perpendicular to it.
Now looking at your two planes, we see that the normals for each plane respectively are (k,2k,5) and (-1, k-1, -7). If the planes are perpendicular to each other, then so should their normals. So, take the dot product of these two vectors and set it to 0:
etc. etc. and solve the quadratic.