Which of the following is(are) the solution(s) to 15x +21= 8?

Can someone me i’m confused. will give brainliest and a whole bunch of points

Two customers went to a post office. first customer paid $12 for 14 and 5 envelopes the second paid 24.80 for 10 postcards and 15 envelopes. what is cost of each envelope?

In the sixth grade 13 out of 20 students have a dog if there are 152 sixth grade students how many of them have a dog