Can you explain please I don’t understand?

don't be alarmed the two lines just mean absolute value, the problem is asking what is m. the problem is asking what number multipyed by 2, subtracted by 1 will give you 3?
2x2=4 right?
so 4-1=3 there!
m=4
so the problem is 2(4)-1=3
i hope i explained well enough!
|2m-1| = 3 is an absolute value equation and as such translates into two separate, different equations. Just suppose that the quantity 2m-1 is already positive; then the absolute value operator has no effect on 2m-1. Thus, in this first case,
2m - 1 = 3, or 2m = 4, or m = 2. This is one of the two solutions of this absolute value equation.
The other case is where the quantity 2m-1 is negative. Applying the absolute value operator to this results in -(2m-1), or -2m+1. We then write -2m+1=3. Subtracting 1 from both sides of this eqn results in -2m= 2, or m= -1. This is the 2nd solution of the absolute value equation.
Thus, your solution set is {-1, 2}.