Answer
$I_1 = 3~mW/m^2$
Work Step by Step
We can find the light intensity when one slit is covered:
$I_{max} = 4~I_1$
$I_1 = \frac{I_{max}}{4}$
$I_1 = \frac{12~mW/m^2}{4}$
$I_1 = 3~mW/m^2$
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