Since Sahil got 28 questions right and Angelina got 7 more wrong answers, therefore Angelina's score is 7 lesser than Sahil. The equation will be 28 - 7 = Angelina's score. And based on the choices, it is a + 7 = 28.
Angelina's score is 21 and that is a.
So, if you substitute a with 21 to the second equation which is:
a + 7 = 28
21 + 7 = 28.