What is force if acceleration is .75? (inclined plane)

[skysunsand]

Homework Statement

A horizontal force F is exerted on a 20kg box to slide it up a 30 degree incline. The friction force retarding the motion is 80N. How large must F be if the acceleration of the moving box is to be .75 m/s^2?

Homework Equations

All the forces on the box in the x and y direction have to equal zero

F=ma
F-Ff-Fn = ma

The Attempt at a Solution

Using F-Ff-Fn,

F-80-20*9.8*sin(30) = 20*.75
F-178= 15
F= 193 N

Which isn't even close to what the answer is supposed to be, which is F=220 N

 

[gneill]

If the box is accelerating then there must be a net force. Newton's first law.

 

[skysunsand]

If the box is accelerating then there must be a net force. Newton's first law.

How does that change my equations?

 

[gneill]

How does that change my equations?

Draw a Free Body Diagram for the box. There must be a net force that produces an acceleration of 0.75m/s² (acting up-slope). So what forces act along the slope?