Please help I would like step by step explanation so I can try to figure out how to do this so I can get a good grade when I take my test!

Answer:
4
Step-by-step explanation:
According to the Triangle Midsegment Theorem, a line segment connecting the midpoints of two sides of a triangle will be half the length of the triangle's third side.
NO = ½ JK
15 = ½ (3x + 18)
30 = 3x + 18
12 = 3x
x = 4