Countries involved in the Mexican revolution

A number of groups, led by revolutionaries including Francisco Madero, Pascual Orozco, Pancho Villa and Emiliano Zapata, participated in the long and costly conflict. Though a constitution drafted in 1917 formalized many of the reforms sought by rebel groups, periodic violence continued into the 1930s.

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