Factor out a GCF of 4, then you are left with perfect squares.
4(100-9x^2)
Factor what is left as a difference of perfect squares (the outside and inside terms from FOIL should cancel out which is why each parenthesis has a different sign
f(10+3x)(10-3x)
7 mph. 28 divided by 4 is 7. Or 7 miles in one hour
Approx - 10%
72 students signed up for canoeing + the 23 students who signed up for trekking + the 13 students who signed up for both = 108 students
there are 120 students total, so you have to subtract 120 - 108 = 12
Now to find the percentage, you have to divide the number of students who didn't sign up for either, by the total
which is .1 = 10/100 = 10%
ANSWER

EXPLANATION
The angle sum theory states that:
This implies that:

Solve for x in the equation above:

That is the value of x.