We know area of triangle = 
In triangle ABC , AB is the base and BC is the height
Now we find the distance between A and B
Distance formula is 
A(0,3), B(7,3)
Distance AB = 
=
= 7
B(7,3), C(7,7)
Distance BC = 
=
= 4
area of triangle = 
area of triangle = 
=
= 14 
It would be $2.25
if each student pays one more penny than the last one the penny count would be up to 150 pennies times the number of student which is 150 so multiply and add your decimal
The coordinates that you record is an example of data from the graph I think
So C/D=.34 And D=.11, now just plug D into the first equation.
C/.11=.34 Divide by .11
C=3.09090909
Now this rounded two decimal places is 3.09!
Hope this helps