x2 + y2 = r2 is the equation for a circle. So for your problem, take the square root of 2250000 to get the radius, in this case, 1500. I am assuming km is the units.
if the orbit of the other is 200km farther out, the radius is 200km greater, so it's radius is 1700km.
you...

When using percentages to multiply 100% is equivalent to a factor of 1, 150% to 1.5, 15% to .15, etc.
first one:
a 60% increase indicates the future yield will be 60% + 100% or 160% of the regular yield. All you have to do is multiply the current yield by 160% or 1.6
3. same...

I would first take care of everything in the parentheses. Expand the first term: multiply the 5 by the 2x and then by the -1/2. The second set of parentheses can just be removed: there is nothing that it has to be multiplied by.
Then I would group all the terms with x together and...