A2 physics help circular orbit

You've got radius
You know solar mass
I assume you know the value of newton's constant
In circular motion the gravitational force of sun is the centripetal of earth
Your circular equation is a bit dodged though

mv^2/r=GMm/r^2

do i rearrange for V?

yup

ok i got v=root GM/r

the i subbed in values
6.67x10^-11 x 5.97x10^24 divided by the radius in m 1.5x10^11
and rooted and i got 51.52
but the answer should have been 3.0 × 10^4ms–1

I got ~29.5 kms^-1; something you should memorise actually it shows up fairly frequently iirc. Anyway, your M is wrong. What's that 5.97? The mass of the sun is 1.989x10^30 kg.

ah i was using the earth's mass, how do you know when to use the sun's mass or earth's, i assumed it was the earth since the question said earth's speed

You just need to know the equations and the physics behind them. If you know that, you'll know which numbers to push through them.

I.e. in this case you did the force on the earth from the sun
$\frac{GM_\odot}{r^2}\times {m_\oplus}$

is equivalent to the centripetal force on the earth necessary for circular motion
$\frac{{m_\oplus}{v^2}}{r}$

cancelled out the and put in your values for $G,M_\odot,r$ which are ~6.67e-11,1.989e30,1.5e11/astronomical-unit.

Could you also please help me with this q?
the answer is A
how do you go about these type of question

Two satellites, P and Q, of the same mass, are in circular orbits around the Earth. The radius of
the orbit of Q is three times that of P. Which one of the following statements is correct?
A The kinetic energy of P is greater than that of Q.
B The weight of P is three times that of Q.
C The time period of P is greater than that of Q.
D The speed of P is three times that of Q.

If it's a new Q I recommend making a new post (makes it easier for future people looking for the same answer)