Explain this to me please.

\(\frac{m-3}{4}=\frac{m+1}{3}\)

[m - 3] / 4 = [m + 1 ] / 3 cross-multiply

3 [ m - 3 ] = 4 [ m + 1] simplify

3m - 9 = 4m + 4 add 9 to both sides, subtract 4m from both sides

-1m = 13 divide both sides by -1

m = -13