The coefficient of x in the equation is 1/(300 grams). Multiply by the reciprocal of that, which is (300 grams).
x = (300 grams)×(2 grams)/(100 grams) = (300×2/100) grams = 6 grams
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This is the way you solve any equation where x is multiplied by some value in the only term on one side of the equal sign.