BJT Circuits at DC, base current calculation
I assume VBB to be the +5V supply and VB the actual base voltage. In that case, the last line contains two unknowns: VE and VB. As the transistor is in active mode, you get a diode drop (rule of thumb: 0.7V) between VE and VB, so substitude VE by (VB-0.7V) or VB by (VE+0.7V) and you can solve the equation.