Hi there!
Recall Newton's Second Law:
[tex]\Sigma F = ma[/tex]
∑F = Net force (N)
m = mass (kg)
a = acceleration (m/s²)
The block will be experiencing two forces; a tension force in the direction of its acceleration (+) and a friction force (-).
[tex]\Sigma F = T - F_K[/tex]
The equation for kinetic friction force is:
[tex]F_K = \mu mg[/tex]
Using Newton's Second Law:
[tex]ma = T - \mu mg[/tex]
Plug in the givens and solve for 'T':
[tex]T = ma + \mu mg\\
\\
T = m(a + \mu g)\\
\\
T = 20(3 + 0.4(9.8)) = \boxed{138.4 N}[/tex]
