Method
15 = - 15 - 8u Add 15 to both sides.
15 + 15 = - 15 + 15 - 8u. This step is meant to get rid of -15 on the right side.
30 = -8u Divide both sides by 8
30 / -8 = u
Note
Usually what you are trying to do is reduce an equation so that it tells you what 1 of the letters is. Since there are many things u could be, you have to choose the one that best fits your answer
u = - 3 6/8
u = - 3 3/4
u = - 3.75
u = - 30/8
u = - 15 / 4