Astudent pulls on a 20 kg box with a force of 50 n at an angle 45 degrees relative to the horizontal. the box increases speed at a rate of 1.5 m/s2. what is the approximate value of the friction force on the box

For this problem the figure below shows the representation of a student who pulls on a 20kg box. We know this variables:

Weight of the box = 20kg
Force used by the student to pull on the box = 50N (This is the tension T)
Angle relative to the horizontal = 45 degrees
Aceleration of the box = 

The figure also shows the Free-Body diagram, Applying Newton's Second Law we can find the equation for this diagram, related to the x-axis as:



Isolating :



That is the friction force on the box.