Answer:
In triangle SHD and triangle STD.
[Side]
Since, a line is said to be perpendicular to another line if the two lines intersect at a right angle.
⇒ 
[leg] [Given]
Reflexive property states that the value is equal to itself.
[Leg] [Reflexive property]
HL(Hypotenuse-leg) theorem states that any two right triangles that have a congruent hypotenuse and a corresponding congruent leg are the congruent triangles.
then, by HL theorem;
Proved!
X represents the number of minutes.
y represents the price of the bill.
Plan A
y = 25.75+0.75x
Plan B
y= 20.99+x
Plan A is better when it is more than 19 minutes.
Plan B is better when it is less than 19 minutes.
The answer would be
X=-2y
Move all the terms that don't contain x to the right side and solve.