friction formula problem?

[pinky2468]

I am having trouble getting this formula to work and I know it is easy! I am having a momentary lapse in math knowledge!
M1=571.4g M2=550g a=.155m/s(sqr) find the force of friction using a=(M1-M2-Mf)g/(M1+M2) or is it M1g-M2g-f=(M1+M2)a
I am having problem getting f by itself, any help?

[Ba]

((M1+M2)a/g)-M1+M2=-Mf multiply the denominator on both sides, divide by g then use addition to isolate Mf

[Pyrrhus]

I am having trouble getting this formula to work and I know it is easy! I am having a momentary lapse in math knowledge!
M1=571.4g M2=550g a=.155m/s(sqr) find the force of friction using a=(M1-M2-Mf)g/(M1+M2) or is it M1g-M2g-f=(M1+M2)a
I am having problem getting f by itself, any help?

a = \frac{(M_{1}-M_{2}-F_{f})g}{M_{1}+M_{2}}

a(M_{1}+M_{2}) = M_{1}g-M_{2}g-F_{f}g

\frac{a(M_{1}+M_{2}) - M_{1}g + M_{2}g}{-g}= F_{f}