Answer: 281.25 milligram
Step-by-step explanation:
Given
Milligrams of aspirin left after t hours is given by

for 

1, 3, 9, 27, 81, 243, 729, 2187, 6561, 19683, 59049, 177147, 531441, 1594323.
If this is the proper sequence pattern, we add up all the numbers to get a sum.
The total sum is 2391457.
The slope of line PQ is m1 = ( 3 - (-3) ) / (2 - (-2) ); then m1 = 6/4 = 3/2;
In the same way, the slope of line RS is m2 = -7/5;
But (3/2) x (-7/5) = -21/10, which is diferrent from - 1;
So, the lines aren't perpendicular.