It's looking for the solved answer, not the simplified expression (no variable in final answer)

Answer:
BC = 144 units
Step-by-step explanation:
BC = BD + DC , substitute values
8x = 4x + 10 + 3x + 8 = 7x + 18 ( subtract 7x from both sides )
x = 18
Thus
BC = 8x = 8 × 18 = 144 units