In triangles ABC ab = 21cm BC = 18cm find the side AC if the perimeter is 58 cm.

Find the length of the side AC of the triangle ABC.
To do this, consider what is given.
By condition, the lengths of the two sides of the triangle ABC, the sides AB and BC, as well as its perimeter, are given.
Let’s write down how to find the perimeter of a given triangle ABC.
In order to find its perimeter, add the lengths of all sides of the triangle.
Thus, the perimeter P of this figure is:
P = AB + BC + AC.
Let us substitute the given tasks into the created equality:
58 = 21 + 18 + AC.
58 = 39 + AC.
Then we solve, as a usual equation:
AC = 58 – 39.
AC = 19 (cm).
Answer: 19cm is the length of the speaker.