Part (a)
Refer to the diagram below to see the two triangles separated.
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Part (b)
If the 3 meter portion is perpendicular to the horizontal portion, then the triangles are similar. We can prove this using the AA similarity theorem. One pair of angles would be the shared overlapping 25 degree angles. The other pair of congruent angles would be the two 90 degree angles. We only need two pairs of congruent angles to use AA. You could use three pairs (and the aptly named AAA theorem) but that's overkill in my book.
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Part (c)
There are a few ways to approach this, but one way is shown below
y/3 = (8.4+6.6)/(6.6)
y/3 = 15/(6.6)
6.6y = 3*15 .......... cross multiply
6.6y = 45
y = 45/(6.6)
y = 6.81818 approximately
When rounding to one decimal place, we get y = 6.8