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Differential Equation involving mechanics

Hi

I need help with the following question. I have attached my question. I need help with part b)

this is what i have managed to do so far, but i don't think i am doing it correctly.

$\displaystyle m\ddot{r}=\frac{mMG}{r^{2}}$

dividing both sides my $\displaystyle m$

$\displaystyle \ddot{r}=\frac{MG}{r^{2}}$

$\displaystyle \int\ddot{r}dt=\int\frac{MG}{r^{2}}dt$

$\displaystyle \dot{r}=\frac{MG}{r^{2}}t+C

$

any help is appreichated

thanks

(Happy)