6x²-6xy+14y²=45

write it in rotated x´y´-system (prime or whatever)

please list it out in steps.

i understand the first step, but i do not understand how to get Θ.

cot2Θ=(A-C)/B

so: cot2Θ=(6-14)/-6 =(-8)/-6 = 4/3

so how can you do: cot2Θ=4/3 !!!!!!!