ElectricPavlov

That is the equation used to describe a straight LINE on the coordinate plane.

4184 j/(kg k)  x  5 k  1kg  = 20920 j   ( the k and the kg 'cancel' ...leaving only 'j' )

Your first equation is incorrect....   you have   3 = 0

You can set it up as a RATIO:

21/100 = 48.3/x    CROSS multiply

21x = 4830            Divide by 21

x= 230  

-4 x 1/2  =  -4/1  x 1/2  =  (-4 x1)  / (1 x 2)  =  -4/2 = -2

Use the calculator !   It will work for you....

Pick any number and multiply the 5 and the 3 by that number and they will be equivalent

5:3

10:6

15:9

20:12      etc, etc...

When they intesect, they are EQUAL.....so, let's set them equal to each other and solve for the 'x' coordinate:

-2x^2 + 4x = 2x-5

-2x^2 +2x+5 = 0

2x^2 - 2x -5 = 0

Yields x = 2.15  and  -1.16

Substitute ino the easiest equation to find the 'y' values

y= 2x-5

y= - 0.7   and  -7.32    

so x,y    =   2.15, -0.7     and   -1.16,-7.32

400km  /  28 l    =  14.28 km/l

x = smallest number

x+1 = next consecutive number

x+2 = third number.....   they all add up to 72

x  +  x+1  +  x+2 = 72

3x+3=72

3x=69

x=23         and 24 and 25