Isolate the k. Note the equal sign. What you do to one side, you do to the other.
First, subtract 2m from both sides
8k + 2m (-2m) = 3m (-2m) + 4
8k = m + 4
Next, divide 8 from both sides
(8k)/8 = (m + 4)/8
k = (m + 4)/8
k = m/8 + 1/2
k = m/8 + 1/2 is your answer
hope this helps