If the sled has a mass of 7.6 kg, what is the mass of your little sister

Please Help! I am having trouble with this one physics problem.

So here's the problem:

You are pulling your little sister on her sled across an icy (frictionless) surface. When you exert a constant horizontal force of 108 N, the sled has an acceleration of 1.7 m/s2 If the sled has a mass of 7.6 kg, what is the mass of your little sister? (answer is in kg)

Well, we know

F = m * a

and m = [ m(sled) + m(sis) ]


Substitute m into the equation, solve it so you have m(sis)= whatever

Then put in the numerical values

56