Since it’s a triangle it is 180
And since it is a right angled triangle the unknown angle is 90
So
8x + 4x + 6 + 90 = 180
12x + 96 = 180
12x = 180 - 96
12x = 84
Divide both side by coefficient of x which is 12
x = 7
Check:
8(7) + 4(7) + 6 + 90 = 180
56 + 28 + 6 + 90 = 180
180 = 180
So therefore x is 7