Given a-2b=8, 4a+3b= 21; prove b= -1

please give a step by step answer

Which mathematical statements are true? 1) if 3 is an odd number, then 3 times 3 is an even number. 2) if 6 is less than 7, then 4 is greater than 7. 3) six is divisible by 3, and 10 is a multiple of 2. 4) the average of the data is greater than the largest value in the data, or it’s less than the largest value in the data. 5) the slope of a linear graph is its rate of change, and the graph’s y-intercept is the initial value. 6) if an equilateral triangle has equal angles, then all its angles will measure 45°.

Simplify the expression. -2/5(10+15m-20n)

The sum of the speeds of two trains is 720.2 mph. if the speed of the first train is 7.8 mph faster than the second train, find the speeds of each.