Please help me with my Trigonometry?

Assuming the point is at (r, Θ) = (2, 7π/4), that's one set of coordinates in the polar coordinate system. Remaining in the polar coordinate system, other sets of coordinates could be determined using the property of coterminal angles that the terminal lines of coterminal angles drawn in standard position are in the same location. Where k = any integer, Θ is coterminal with Θ ± 2kπ

(r, Θ) = [2, (7±8k)π/4]

= ..., (2, -17π/4), (2, -9π/4), (2, -π/4), (2, 7π/4), (2, 15π/4), (2, 23π/4), ...

or

(r, Θ) = [-2, (1±8k)π/4]

= ..., (-2, -7π/4), (-2, π/4), (-2, 9π/4), ...

You can choose any two of the above polar coordinates to have two sets of polar coordinates that refer to the illustrated point.

If you want to depart the polar coordinate system and enter the Cartesian (rectangular) coordinate system, any of the (r, Θ) coordinates can be converted to rectangular coordinates with the transformation:

y = rsinΘ, x = rcosΘ

For example:

(r, Θ) = (2, 7π/4) → (x, y) = (2cos(7π/4), 2sin(7π/4) = (2√2/2, -2√2/2) = (√2, -√2)

Any of the above polar coordinates will transform to the same set of rectangular coordinates, (x, y) = (√2, -√2), so you could choose any one of the polar coordinates above and the rectangular coordinate (√2, -√2) and have two sets of coordinates that refer to the illustrated point.