By Newton's 2nd law of motion, F = ma, where F is force, m is mass, and a is acceleration.
Rearranging this equation to find acceleration would give us: a = F/m
The horizontal force to the right is 10N, because the box is pushed to the right with a force of 20N, and the friction force of 10N opposes that, so: 20N - 10N = 10N
The mass is 2kg.
Putting these values into the equation gives us: a = F/m = 10/2 = 5ms^-2