a)
$\phantom{X}2\sqrt{3}\,+\,5\sqrt{3}\,=\,7\sqrt{3}\,\centerdot\,\sqrt{3}\,=$ $\,21\,+\,\dfrac{\,1\,}{2}\,=\,\dfrac{\,43\,}{2}\phantom{X}$
b)
$\phantom{X}5\sqrt{2}\,+\,6\sqrt{3}\,=\,11\sqrt{5}\phantom{X}$
c)
$\phantom{X}18\sqrt{3}\,-\,6\sqrt{3}\,+\,4\sqrt{3}\,=\,8\sqrt{3}\phantom{X}$
