Use Heron's formula.
A = sqrt{(s - a)*s - b)(s - c)}, where a, b, and c represent the sides of the triangle.
We need to find s first.
So, s = (a + b + c)/2.
s = (13 + 6 + 9)/2
s = 28/2
s = 14
We now plug everything into the Heron's area formula.
A = sqrt{(14 - 13)(14 - 9)(14 -6)}
A = sqrt{(1)(5)(8)}
A = sqrt{40}
A = 6.3245553203
Let us now round to one decimal place as instructed. We round to the tenths place, in other words.
A = 6.3 cm^2
Did you follow?