The powhatan indians who lived near jamestown colony

At the time English colonists arrived in the spring of 1607, coastal Virginia was inhabited by the Powhatan Indians, an Algonquian-speaking people. The Powhatans were comprised of 30-some tribal groups, with a total population of about 14,000, under the control of Wahunsonacock, sometimes called “Powhatan.”

the perimeter of a triangle is 69 cm. side a is 5 cm shorter than side b. side c is twice as long as side a. find the length of each side. i need to know how to do it in 2 equations.  

equations:

a = b-5

c = 2a

a + b + c = 69