in the q it doesn seem to be mentioned wat is the dist of the side parallel to the y axis from the y axis...let us asuume it is 'a'
now for the sides of the loop parallel to x axis,the force will cancel out...cuz @ each pt on the 2 parallel sides,same force acts but in opp directions
now for 2 sides parallel to y axis:
for side closer to Y axis:
B=Bo(1+a/l)
so force=Bo(1+a/l)*i*l towards neg x..
for the side farther away from Y axis
B=Bo(1+(a+l)/l)
so force=Bo(1+(a+l)/l)*i*l towards pos x..
so net force=Bo(1+(a+l)/l)*i*l-Bo(1+a/l)*i*l= Bo*i*l