Question 144840
There are 2 main things to know
1) You have to get {{{k}}} all by itself on one side of the equation
2) Anything you do to one side, you MUST do to the other side
So, first
Subtract {{{3u}}} from both sides
This is the same thing as adding these 2 equations together
{{{-3u = -3u}}}
{{{t = 3k + 3u}}}
Notice that the {{{-3u}}} and the {{{3u}}} on the right side cancel
So, now you have
{{{t - 3u = 3k}}}
Now divide both sides by {{{3}}}
{{{(t - 3u) / 3 = (3k) / 3}}}
On the right side, the {{{3}}} in the denominator cancels the {{{3}}} 
in the numerator
Now you have
{{{(t - 3u) / 3 = k}}}
Now just rewrite the equation in the opposite direction
{{{k = (t - 3u) / 3}}}
And that's the answer