The tennis team is selling tickets to a car wash for 6 dollars when they do not sell very many tickets the team decrease the price 25% what is the new cost of a ticket

For this case what you must do is the following rule of three:
 6 $ ---> 100%
 x > 25%
 Clearing x we have:
 x = (25/100) * (6)
 x = 1.5 $
 Then, the new cost will be
 6 $ -1.5 $ = 4.5 $
 
 the new cost of a ticket is 4.5 $

-2x + 7 = 7

subtract 7 from both sides.

-2x = 0

divide both sides by -2.

x = 0

the value of x is equal to 0.