Answer
$2.51$ times more intense
Work Step by Step
We know that $M=\log\frac{I}{S}$ and thus $10^M=\frac{I}{S}$
Hence here if $I_1,I_2$ are the intensities, we have:
$I_1=S10^{7.2},I_2=S10^{6.8}$
Hence
$\frac{I_1}{I_2}=\frac{10^{7.2}}{10^{6.8}}\approx2.51$ times more intense than Northridge quake.