—second law of newton

where did the “2a” come from in the first equation?

i know that frictional force is equal to “normal force x coeficient of friction” but the problem is with that number 2 on left side.

Consider two closed systems a and b. system a contains 3000 kj of thermal energy at 20°c, whereas system b contains 200 kj of thermal energy at 50°c. now the systems are brought into contact with each other. will the heat transfer occur from system a to system b?