First, simply each term. Then, add the coefficients of the like terms (same radicals).
[tex]3\sqrt{28} = 3\sqrt{2*2*7} = 3(2)\sqrt{7} = 6\sqrt{7}[/tex]
[tex]5\sqrt{63} = 5\sqrt{3*3*7} = 5(3)\sqrt{7} = 15\sqrt{7}[/tex]
[tex]4\sqrt{112} = 4\sqrt{2*2*2*2*7} = 4(2*2)\sqrt{7} = 16\sqrt{7}[/tex]
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[tex]3\sqrt{28} - 5\sqrt{63} + 4\sqrt{112}[/tex]
= [tex]6\sqrt{7} - 15\sqrt{7} + 16\sqrt{7}[/tex]
= [tex](6 - 15 + 16)\sqrt{7}[/tex]
= [tex]7\sqrt{7}[/tex]
