Problem$/$Task $4$

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 magnitude of $0\mathrm{.}4\frac{m}{s^2}.$ The mass of the cart is $400$ grams. Calculate the magnitude of $F_1.$