A. write an expression that represents the following expression using the variable b: "billy's age in 8 years." b. write an equation using the expression above if in 8 years billy will be 21 years old. c. how old is billy now? d. write and solve a word problem that matches the equation: a - 6 = 10.

"Billy's age in 8 years", this refers to a future age, which implies adding 8 years to the present age. The expression would be:

"In 8 years Billy will be 21 years", this expression, using the first one too, would be:

How old is Billy now?

The answer to this question is the solution of the second equation:

Billy is 13 years old now.

A problem for the expression could be:

Six years ago Adam was 10 years old.

The six years in the past refers subtracting, and the age back then is the result of that difference.

i believe the answer is c.