An introduction to equations x-3=10

Can someone explain the elimination method to me for this problem?

Which approach to the theory-and-research cycle does the following sequence reflect: (1) specific observations suggest generalizations, (2) generalizations produce a tentative theory, (3) the theory is tested through the formation of hypotheses, and (4) hypotheses may provide suggestions for additional observations?

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