Barbara walks on a circular trail at the park. The trail has a radius of 45 yards. What is the circumference of the walking trail?

Can someone explain the elimination method to me for this problem?

Mexico city, mexico, is the world's second largest metropolis and is also one of its fastest-growing cities with a projected growth rate of 3.2% per year. its population in 1991 was 20,899,000 people. use the formula p = 20.899e0.032t to predict its population p in millions with t equal to the number of years after 1991. what is the predicted population to the nearest thousand of mexico city for the year 2010?

What is the simplified form of (3.25x10^3)(7.8x10^6) written in scientific notation?