(2) The slope of 4x - ky = 6 can be calculated by re-expressing it in slope-intercept form:

So we see the slope is 4/k.

The slope of 6x + 3y + 2 = 0 can also be calculated by re-expressing it in slope-intercept form:

So we see the slope is -2.

For the lines to be perpendicular, we need

This gives k = 8.