How do you do expressions in maths

Need solving this second acceleration problem.

Marco has $38.43 dollars in his checking account. his checking account is linked to his amazon music account so he can buy music. the songs he purchases cost $1.29. part 1: write an expression to describe the amount of money in his checking account in relationship to the number of songs he purchases. part 2: describe what the variable represents in this situation.

Bradley hikes 12 miles in 3 hours. in relation to bradley’s hike, which statement describes the expression