Same as the last one you asked.
AB + BC = AC. Just think of it like Part + Part = Whole Thing
(2x + 6) + (6x + 5) = 51
Combine like terms:
8x + 11 = 51
Subtract 11 from both sides to get the variable and its coefficient on its own:
8x = 51 - 11
8x = 40
Divide both sides by 8 to isolate the variable:
x = 5
It's asking for us to find BC so now we plug in x = 5 to the equation it gave for BC.
6x + 5
6(5) + 5
30 + 5
35
BC is 35 units long.