Whenever you have a problem like this, you need to figure out the mathematical symbols for the words in the problem.
18 more than is +18
1/3 his brother's weight is 1/3 x b
So 135=18 + 1/3b
Subtract 18 from both sides: 117 = 1/3b
Multiply both sides by 3: 351 = b. His brother weighs 351 pounds.