Explanation:
By the definition of mod function:
IxI = x, if x > 0
= -x if x < 0
Now, simplify Left and right side individually,
LEFT SIDE : I 3+10I = I 13I
as 13 > 0, so I13I = 13
RIGHT SIDE: I3I +I10I
Now,as 3 > 0 and 10 > 0, so I3I = 3 and I10I = 10
hence, I3I +I10I = 3 + 10 = 13
Hence on simplifying the mod function , The left side = the Right side = 13
Hence l3+10l = l3l + l10l