Answer
$m=1.64\times 10^{-27}Kg$
Work Step by Step
We know that
$\lambda=\frac{h}{mv}$
This can be rearranged as:
$m=\frac{h}{v\lambda}$
We plug in the known values to obtain:
$m=\frac{6.626\times 10^{-34}}{0.90(2.9979\times 10^8)(1.5\times 10^{-15})}$
$m=1.64\times 10^{-27}Kg$