Answer:
one, no, one, infinitely
Step-by-step explanation:
a. you can get 7x = 21 - 21 = 0 ==> x = 0
b. subtract 12x from both sides ==> get 15 = -15, which is not true
c. -5x-5x=25+25 ==> -10x = 50 ==> x = -5
d. get -2x -6 -7 = -2x - 6 - 7 ==> so it's infinitely
