A Circle Equation Problem (DSAT Math: BB Test 2, Mod. 2 #6)

The College Board has been replacing some of the questions from the earlier Bluebook Tests (Tests 1-3) with newer, harder questions. This one is the new #6 from Module 2 of Bluebook Test 2.

[fromasmi]

Question: for all this types where two variables are provided should we make the unasked value 1, like how you put p=1 here

[francescamangionecarneiro]

i don't understand why did you put the blank spaces