For a given triangle with sides a, b, c and a corresponding angle, A to side a(opposite side a), we would find the measure of the angle by applying the cosine rule which is expressed as
a^2 = b^2 + c^2 - 2bcCosA
Looking at the given triangle, given that a = 12, b = 7, c = 10 and angle A = x, the formula would be applied as shown below
12^2 = 7^2 + 10^2 - 2 * 7 * 10 * Cosx
144 = 49 + 100 - 140Cosx
144 = 149 - 140Cosx
140Cosx = 149 - 144
140Cosx = 5
Cosx = 5/140
Cosx = 0.0357
x = Cos^(- 1)(0.0357)
x = 87.95 degrees