halp plz on these questions THX

1) A parabola with equation y=x^2+bx+c passes through the points (-1,-11) and (3,17). What is c?

We have that

(-1)^2  + b(-1)  + c  = -11  ⇒   1 -b + c  = -11  ⇒   -b + c  = -12       (1)

and

(3)^2   + b(3) + c  = 17  ⇒  9 + 3b + c  = 17  ⇒  3b + c  = 8  ⇒  -3b - c  = -8    (2)

Add (1)  and (2)  and we get that

-4b  = -20

b = 5

So....using (1)     -b + c   =-12

-5 + c  = -12     subtract 10 from both sides

c  =   -7

The Area of triangle ABC   = (1/2)(B)(AB)

Draw  DE  parallel to CB  such that DE is  at a height of (1/2)AB

We have a trapezoid DEBC....the area of this trapezoid is  

(1/2) (1/2) (AB)(( DE + BC)

But using similar triangles....DE   =  1/2BC

So...the area of  trapezoid DEBC  =  (1/2) (1/2)(AB) ( [1/2 BC] + BC)  =  (1/4)(AB)( 3/2 )(BC)  = (3/8)(AB)(BC)

And if  P is located inside this trapezoid.....the area of triangle PBC will be less than (1/2) that of  ABC

So.....the  probability  is   [ DEBC] / [ABC ]  =  (3/8)(AB)(BC) /  [ (1/2) (AB) (BC) ] =  (3/8)/(1/2)  =  3/4

3) The first six rows of Pascal's triangle are shown below, beginning with row zero. Except for the 1 at each end, row 4 consists of only even numbers, as does row 2. How many of the first 20 rows have this property? (Don't include row 0 or row 1).

The even rows  occur at  the  2^n  rows    where  n is an integer  ≥ 1

So    rows   2, 4, 8, 16    have the property  that all the entries except fo the  beginning and ending 1s  are even

THX FOR ALL OF THE HELP. These questions had me stuck for a long time. THX