6 is easy
7. never divide by a fraction, flip ot upside down and multiply, so multiply by 5 on both sides
8. subtract 3.45 on both sides
9. the answer is just 1
If we want to know the greatest common factor, we should ask: is one of the numbers divisible by the other? if yes, then the smaller number will be the answer!
let's try:

so, yes!
6 is the greatest common factor of the two:)
Check the picture below.
now, we know that the slanted legs are congruent, since it's an isosceles trapezoid, we also know that the bases are the parallel sides, so, the "altitude" or distance from those bases are the same length, for each of those triangles in the picture.
now, the bases are parallel, that means the altitude segment is perpendicular to the base, the longest side at the bottom, so, we end up with a right-triangle that has a Hypotenuse and a Leg, equal to the other triangle's.
thus, by the HL theorem for right triangles, both of those triangles are congruent, and if the triangles are congruent, all their sides are also, including the ones on the base.