Use the Change of Base formula to rewrite $\displaystyle{\log}_{{{2}}}{50}\usepackage{cmbright}\Huge\$ using natural logarithms:

$\displaystyle{\log}_{{{2}}}{50}=\frac{{{\ln{{50}}}}}{{{\ln{{2}}}}}\usepackage{cmbright}\Huge\$

Use the given values to approximate the quotient: $\displaystyle\frac{{{\ln{{50}}}}}{{{\ln{{2}}}}}\approx\frac{{{3.9}}}{{0.7}}={5.6}\usepackage{cmbright}\Huge\$