Please answer the following question:

Two forces are applied to a cart as shown in the

figure on the left $($hands are pulling on the strings

attached to a cart$).$ Force $F_2$ has a magnitude of $6N.$

The cart is moving to the left and speeds up with an

acceleration that has the maganitude of $0\mathrm{.}4\frac{m}{s^2}.$ The

mass of the cart is $400$ grams. Calculate the magnitude of $F_1.$