So 48 and (3x+12) are congruent by Alternate Exterior Angles, so all you need to do is equal them to each other and solve.
[tex] 48=3x+12 [/tex]
Firstly, subtract 12 on each side: [tex] 36=3x [/tex]
Then divide both sides by 3, and your answer should be [tex] x=12 [/tex], or B.
