In scientific notation, it would be 2x10^3.
An easy trick to help remember when it is a multiple of 10 is that you do the numbers that aren't zero (2) times 10 and the exponent is how ever many zeros there are after the other numbers (3).
Y = kx is the direct variation equation with k being the "constant of variation"
The constant here being the conversion decimal .91
y = 0.91x
You are right. In circle equation we have =r² so the correct answer will be C od D. But we have to calculate the center of a circle.

Complete the square:

And we have:

So the answer is C (the same left side of equation).