0.000000591 in scientific notation

Acar started driving from city a to city b, distance between which is 620 miles, at a rate of 60 mph. two hours later a truck left city b and started moving towards city a at a rate of 40 mph. what distance from city a is the point where the truck and the car will meet?

Prove that the value of the expression 7^8–7^7+7^6 is divisible by 43.

What does independent variable and dependent variable mean in math?