Newton's Second Law Practice Test

In the lab frame, a $2.0,\text{kg}$ cart on a level track is pulled right by a string with tension $8,\text{N}$. Kinetic friction on the cart is $3,\text{N}$ left. The cart accelerates right. If the same forces act on a $4.0,\text{kg}$ cart, what is the new acceleration?

In the ground frame, a cart is pushed right by $15,\text{N}$ while friction is $9,\text{N}$ left, producing a rightward acceleration of $1.5,\text{m/s}^2$. What is the cart’s mass?