I don't know if I understand this question that well, but if I am right, the first digit of the question you are asking is 5, and it is in the hundreds place. If this is not what you mean, then can you put more description in this question please?
Answer:
The number is 3.
Step-by-step explanation:
To find : A number which decreased by 12 equals 3 times it’s opposite ?
Solution :
Let the number be 'x'.
It's opposite is '-x'.
A number which decreased by 12 equals 3 times it’s opposite
i.e. 
Solving the equation,
Therefore, the number is 3.
What is the series you are referring too?
Part a : you could draw a line from each corner to the middle
This would give you 7 triangles in total
Part b : area of one triangle : 4.5*7.8 = 35.1 / 2 = 17.55in <-- area of 1 triangle
Part c : so if there are 7 triangles and the area of 1 triangle is 17.55, you would do:
7 * 17.55 = 122.85
Complementary angles are two angles that add up to 90°
Supplementary angles are two angles that add up to 180°
Vertical angles are two angles that are opposite of each other and are also congruent
