Alan

The intersection refers to those elements that are common to both sets, so $A\cap B={2, 7}$ 

As follows:

I'll leave you to do the number crunching.

Steps as follows:

It's a method for solving simultaneous linear equations that have been expressed in matrix form.  See https://www.youtube.com/watch?v=ajJD0Df5CsY 

"In the figure, we have a square and a circle inside a larger square.

Find the radius of the circle, to 3 decimal places"

"The sinusoidal function f(x) = 33sin (7x − π/12) + 33 models the height, in centimeters, that a valve stem of the a tire is above ground x seconds after the tire begins to rotate.

How long does it take the valve stem to complete one revolution?"

I'll let you turn it into the required decimal.

If u >= -1/2 then 2u is not in the interval 

If u <= 1/20 then -20u is not in the interval

So we must have  u in the interval [-1/2, 1/20]

deleted..

A is the amplitude, which is half the vertical distance between the peak (86) and the trough (38).

The period, T, is 12 months.

The vertical shift is the y-value about which the sine curve oscillates i.e. it is the "middle" of the curve in the  vertical direction.

The offset angle of pi is there because the function starts going in the negative y-direction rather than positive y-direction.

Does this help?