Answer
$\sqrt {a^2+b^2}$ units
Work Step by Step
Use the formula $d(P_1,P_2)=\sqrt {(x_2-x_1)^2+(y_2-y_1)^2}$:
$d((a,b),(0,0))
\\=\sqrt {(0-a)^2+(0-b)^2}
\\=\sqrt {(-a)^2+(-b)^2}
\\=\sqrt{a^2+b^2}$
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