recall that the sum of all interior angles in a triangle is 180°, therefore F + U + N = 180°, and thus
[tex]\bf \stackrel{F}{(x+21)}~~+~~\stackrel{U}{(x+23)}~~+~~\stackrel{N}{(x+61)}~~=~~180
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3x+105=180\implies 3x=75\implies x=\cfrac{75}{3}\implies x=25\\\\
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\measuredangle N=(25)+61[/tex]
