Hey there.
4(2.1k - 4) = 6(3.4k - 2); apply the distributive property to remove the parenthesis.
8.4k - 16 = 20.4k - 12; Get the variable to one side, so subtract 8.4k.
-16 = 12k - 12; isolate the variable by adding 12.
-4 = 12k; divide by 12 to solve for our variable.
k = -1/3
I hope this helps!