Since the two forces are on the same direction but opposed verse, we can subtract the vectors so that the overall force on the object will be 60 N - 20 N = 40 N to the left. We can then use Newton's second law which states

We can find the acceleration by dividing the force applied to the object with the mass of the object, therefore:
a = 40 N / 10 kg = 4 m/s²