STEP - BY - STEP EXPLANATION
What to find?
The number of players on both teams.
Given:
Total number of athletes = 61
Number of players on football team = 49
Number of player on baseball team = 18
Let x be the number of players on both teams.
Step 1
Draw a Venn diagram representing the information.
Step 2
Add and equate to the total number.
That is;

Step 3
Collect like term.

ANSWER
There were 6 players on both teams.