First step:
Get k by itself!
The opposite of adding is subtracting so u need to subtract 4 from both sides in order to get k by itself 
K + 4 = 7
 - 4 - 4
Second step: 
Since 4 - 4 = 0, it cancels out. Now k is by itself and now you need to subtract 7 by 4 
7 - 4 = 3 
Answer:
K = 3