For the first one c-9=(2)(4)+6
You are gonna multiply 2 and 4 first, 2*4=8. Now you have c-9=8+6. 8+6=14. So now the equation is c-9=14. Now we are going to add 9 to both sides. So c=23
For the second equation 10+y=90. We are going to subtract 10 from both sides. So, y=80.
Third, add 4 to both sides. a=10.
Fourth, subtract 8 from both sides. d=-9
Fifth, 5+2(3+4)=-x.
First we are going to add 3 and 4. 3+4=7. 5+2(7)=-x. Next we are going to multiply 2 and 7. 2*7=14. 5+14=-x. 5+14=19. So our equation is 19=-x. Next we are going to multiply both sides by -1. Making our final equation x=-19.
c=23
y=80
a=10
d=-9
x=-19
I hope that helps, if you need any further explanation just ask.<span />
Answer:

Step-by-step explanation:
The formula of a slope:

If
m > 0, then a line is rising
m < 0, then a line is falling
m = 0, then a line is horizontal
m is undefined, then a line is vertical
<h2>a.</h2>
(2, 1) and (4, 5)

<h2>b.</h2>
(-1, 0) and (3, -5)

<h2>c.</h2>
(2, 1) and (-3, 1)

<h2>d.</h2>
(-1, 2) and (-1, -5)

The mathematical sentance would be, 740 - 14 = 726, hope this helps.:D