Answer:
 a = 3 m/s²
Step-by-step explanation:
given,
mass of crate = 20 Kg
horizontal force on crate = 70 N
frictional force on the crate = 10 N
acceleration of crate = ?
now, calculating net force acting on the crate.
 F = horizontal force - frictional force
 F = 70 - 10
 F = 60 N
net force on the crate is equal to 60 N.
We also know that
 F = m a
 60 = 20 x a
 a = 3 m/s²
Hence, the acceleration of the crate is equal to 3 m/s²