Comparing gravitational acc at surface of 2 diff planets

[Maiia]

Homework Statement

Planet X has five times the diameter and
seven times the mass of the earth.
What is the ratio gX : ge of gravitational
acceleration at the surface of planet X to the
gravitational acceleration at the surface of the
Earth?Well, I used the equation:
$$\frac{Fplanetx}{FplanetE}$$= $$\frac{Mx}{Me}$$ * ($$\frac{Rx})({Re}$$)²
i decided to use variables:
Mx= 7Me
Rx= ((2Re)*5)/2= 5
plugging in:
7Me/Me(25Re)
i got 175...but none of the answers have that big of a number
I would really appreciate it if someone could help me figure out what I did wrong.

 

[rock.freak667]

You should get
$$\frac{g_x}{g_E}= \frac{7GM_E}{25R_E^2} \times \frac{R_E^2}{GM_E}$$

 

[Maiia]

ohh i see what i did wrong.. Thanks for your help! :)