The sides are proportional in similar triangles, so $\displaystyle\frac{{{A}{B}}}{{{X}{Y}}}=\frac{{{A}{C}}}{{{X}{Z}}}\usepackage{cmbright}\Huge\$ . You get $\displaystyle\frac{{4}}{{n}}=\frac{{3}}{{5}}\usepackage{cmbright}\Huge\$ by substituting the given values into the equation.