i also need help with these two questions:

Question

1/r=1/r1 +1/r2
solve for r1

and:
f= G(mM/r^2); solve for m

Answer 1

would you have trouble solving

1/2 = 1/x + 1/3 ?

Probably not. So, follow the same steps here:

1/r = 1/r1 + 1/r2
1/r1 = 1/r - 1/r2
1/r1 = (r2-r)/(r*r2)
r1 = (r*r2)/(r2-r)

The other one is even easier:

F = GMm/r^2 = (GM/r^2)m
m = F(r^2/GM) = Fr^2/GM