Vitamania

Notice that the units of the given quantities aren't same, so we'll have to convert them into a same unit, thereafter it's just a game of simple math

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It's given that butter contains 5.5 ㎍ riboflavin per gram, therefore

$1 g$ butter contains $(1)*(5.5*{ 10 }^{ -6 })g$ of riboflavin

$2 g$ butter contains $(2)*(5.5*{ 10 }^{ -6 })g$ of riboflavin

$3 g$ butter contains $(3)*(5.5*{ 10 }^{ -6 })g$ of riboflavin

Similarly $xg$ butter will contain $(x)*(5.5*{ 10 }^{ -6 })g$ of riboflavin

This $xg$ of butter will be able to meet the person's daily requirement of riboflavin, which is 2.0 mg or $2.0*{10}^{-3} g$

In mathematical terms this can be written as

$x*5.5*{10}^{-6} g = 2.0*{10}^{-3} g$

$x=\frac{2.0 \times {10}^{-3}} {5.5 \times{10}^{-6}} = \frac {4000} {11} = 363.63 g$

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Always keep these conversions in mind

$1 mg$ = $1*{ 10 }^{ -3 }g$ and $1 \mu g$ = $1*{ 10 }^{ -6 }g$

The units must propagate through any calculations to be in grams ( of butter [B] ) as the question asks. Convert all given quantities into grams.
$2.0mg[B_{2}] = 2.0 \times 10^{-3}g[B_{2}]$ and $\frac{5.5 \mu g [B_{2}] }{1g[B]} = \frac{5.5 \times 10^{-6} g [B_{2}] }{1g [B] }$
Letting $N$ represent how many grams of butter would have to be taken per day to get the required amount of riboflavin in units ( g [B] )
$\frac{5.5 \times 10^{-6} g [B_{2}] }{1g [B] } \times N = 2.0 \times 10^{-3} g [B_{2}]$
$N= \frac{2.0 \times 10^{-3} g [B_{2}]}{\frac{5.5 \times 10^{-6} g [B_{2}] }{1g [B] }} = \frac{4 \times 10^{6}}{11 \times 10^{3}} g[B] = \frac{4}{11} \times 10^{3} g [B] \approx 363.64 g [B]$