It seems that you made a mistake in your first step, trying to isolate the term $\displaystyle{4}{R}\usepackage{cmbright}\Huge\$ . You cannot do this by subtracting T. You need to add T to both sides. The correct answer is $\displaystyle{R}=\frac{{{K}+{T}}}{{4}}\usepackage{cmbright}\Huge\$ .