helpp in b) and c)

[tawdry birch]

Here's the question

[blissful beacon]

1. Wait patiently for a helper to come along.
2. Once someone helps you, say thank you and close the thread with:

+close

3. Feel free to nominate the person for helper of the week in #helper-nominations
4. Do not ping the mods, unless someone is breaking the rules.
5. If you're happy with the help you got here, and the server overall, you can contribute financially as well:

[fiery bear]

let's write the rotation matrix using simultaneous equations.
x' denotes the image of x after transformation
$x'= x*\cos(\theta)-y*\sin(\theta)$ when rotated 90 degrees, x is replaced with -y.
$y'=x*\sin(\theta)+y*\cos(\theta)$
the y is replaced with x under the same rotation.
In general this matrix represents 2d rotations

[chrome ibex]

Apenguinarmy

[fiery bear]

Then you can evaluate the matrix for a specific angle of rotation

[tawdry birch]

+close