When you get a problem like this, always start off with what the question is asking for. In this case, we're looking at acceleration. We see a lot of forces involved, so it's safe to say we're going to need Newton's second law:
Mass is given to you, but force is the "net force." We'll need to combine all of the forces on the block to find the net force.
Start off by diagramming the forces on the block. In the vertical axis, we have to worry about two forces: gravity downward (F(gravity)=mg) and the applied force (800N upwards). Because we're not moving the cement block sideways, we don't have to worry about friction or any other left-right forces.
In case you're worried about axes, let's just say that "up" is positive. Because of this, we can now say which force is negative and which is positive. Because our applied force is going "up," it will be a positive 800N. Because gravity is going "down," it will be -mg.
Our net force can now be found as follows by combining the two forces we know are acting on the block:
`F(net) = F(gravity)+F(applied)`
`F(net) = -mg + 800N`
Setting `m = 50 kg` and `g = 10 m/(s^2)`:
`F(net) = -(50)(10) + 800 = -500 + 800 = 300N`
We now know the net force acting on the block is 300 N upwards. Now we can use Newton's Second Law to solve for the acceleration:
`F(net) = ma`
`300 = 50a`
`a = 6 m/(s^2)`
And there you have it!