Find missing side of triangle

A triangle has sides of length 'a' cm, (25 - a) cm and (2a +3) cm. If the perimeter is 40 cm, find the length of 'a' cm.

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Perimeter is distance around an object.
To find perimeter, add up all the sides.

a + (25 - a) + (2a + 3) = 40   {perimeter equals all sides added together}
a + 25 - a + 2a + 3 = 40   {removed parentheses}
2a + 28 = 40   {combined like terms}
2a = 12   {subtracted 28 from each side}
a = 6   {divided each side by 2}

a = 6 cm

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