Help Me Please. Explain if possible, I just don't understand.

Evaluate the expression for the given value of the variable 7a - 4a for a =8

Tony is charged $ 50 and additional $0.15 per miles for renting a car. a) represent the cost of renting a car with an equation, and the determine the cost if he drove it70 miles. b) what would be the cost of a car rental if the car was driven 250 miles? show work

I'm marking the brainliest and it's 15 points