2x + 3x + 20 + 10x - 5 = 10x + 16 What does x equal?

Hel and a good explanation, not just the answer pls.

Jamie is hiking up a small mountain. he climbs up at a constant rate of 300 feet/hour until he reaches the peak at 1,500 feet. after that, he hikes down at the same rate to the base of the mountain. the equation that models jamie’s elevation, e, after t hours is e = . jamie’s elevation will be 600 feet after hours and after hours.

Kylie explained that (-4x+9)^2 will result in a difference of squares because (-4x+9)^2 (-4x+9)^2 =(-4x)^2+(9)^2=16x^2+81

What is the number 321,000,000 in scientific notation? also what is the number? ?