One fifth of a number plus five times that number is equal to seven times the number less 18. find the number.

OK.  First we have to figure out names for all the numbers you mentioned.

The mysterious number . . . . . call it Q
One fifth of the number . . . . . 0.2 Q
Five times the number . . . . . . 5 Q
Seven times the number . . . . 7 Q

(One fifth plus five times the number) . . . 5.2 Q
(Seven times the number less 18) . . . . . . 7 Q - 18

You said that these last two things are equal, 

so I can write . . . . . . . . . . . . . . . . . .  7 Q - 18  =  5.2 Q

Now, subtract  7Q  from each side . . .  -18 = -1.8 Q

Divide each side by  -1.8 . . . . . . . . . .  10 = Q 

the formula of a slope:

from a table we have

substitute:

the slope of f(x) is -1.

the graph of funcfion g(x) is a horizontal line. the slope of a horizontal line is equal 0. therefore the slope of g(x) is 0.

-1 < 0

answer: the function g(x) has the greatest slope.