The student did not distribute the negative when subtracting. Distributing the negative, the expression is equal to 6 b + 4 r – 2 b – r . Then use the commutative property to rewrite it as 6 b – 2 b + 4 r – r . Then you can use the distributive property to write as (6 – 2) b + (4 – 1) r , which simplifies to 4 b + 3 r .