36j + j = 37j
Explanation.
You could translate this sum into a normal sebtence: I have 36 apples and add 1 apple more.
The sum will then be 36 apples + apple = 37 apples.
Replacing the word (in this explanation 'apple') by a letter learns us 36a + a = 37a
Hence 36j + j = 37j
