We are given the equation

In order to make k the subject, we will have to isolate k and express it in terms of the other variable, t. Let us begin:
Multiply both sides by 3t

Let us now subtract 4k from both sides:
Take k out as the common factor on the left hand side of the above equation:

Divide both sides by 21t-4

If we multiply the numerator and the denominator on the right side by -1 we will get, k to be as:

The above the given equation expressed with k as the subject.