Question 303677
Express the first, second, and third angles of the triangle be expressed as x, y, and Z. From the first sentence x=12. From the second sentence y=5z. For triangles the sum of the angles of a triangle is 180. So we get three linear equations

x = 12
y = 5z
x+y+z = 180

Put the expressions for x and y fron the first and second equations into the third equation to get

12+5z+z = 180
6z = 180-12
6z = 168

So the third angle measurement, z, is

z=28.