Answer:
The first one
Step-by-step explanation:
7 + 21 = 28
7( 1 + 3 ) = 7 x 4 which is 28
since they are the same answer it's the first one
Why the others don't work:
3( 3 + 7) = 3 x 10 which is 30 not 28
7( 3 + 3) = 7 x 6 which is 42 again not 28
finally 3( 1 + 7) = 3 x 8 which is 24, and as you prob guessed, not 28.