To solve the problem, we need to determine how many cans are currently on the shelf, given that the shelf can hold a total of 58 cans and that Alexei can still put 39 cans on the shelf before it is full.
Let's define a variable:
- Let \( c \) be the number of cans already on the shelf.
According to the problem, the equation can be set up as follows:
\[
c + 39 = 58
\]
This equation means that the number of cans already on the shelf (c) plus the 39 additional cans he can still add equals the full capacity of the shelf (58 cans).
Now, we can solve this equation for \( c \):
\[
c + 39 = 58
\]
Subtract 39 from both sides:
\[
c = 58 - 39
\]
\[
c = 19
\]
So, the number of cans already on the shelf is 19.
Thus, the correct equation and answer are:
\[
c + 39 = 58 \quad \text{and} \quad c = 19
\]