Answer
$F_f = (-4.0~N)~\hat{i}$
Work Step by Step
We can use Newton's Second Law to find the frictional force $F_f$:
$F_{app}+F_f = ma$
$F_f = ma-F_{app}$
$F_f = (10~kg)(0.60~m/s^2)-(10~N)$
$F_f = -4.0~N$
We can express the frictional force in unit-vector notation:
$F_f = (-4.0~N)~\hat{i}$
