I will see that 12x-6 will equivalent to 90° because both angle add together to get 180° and each of them has 90° each which is complementary angel, so here is the equation:
12x-6=90
Add 6 to each side
12x-6+6=90+6
12x=96
Divided both side by 12
12x/12=96/12
x=8
To prove x is correct, I will give you another way to et the same answer. We see that 12x-6 and 90° formed the straight line which add up to total of 180°, so
12x-6+90=180
12x+84=180
Subtract 84 to each side
12x+84-84=180-84
12x=96
Divided both side by 12
12x/12=96/12
x=8. As a result, the correct answer is 8. Hope it help!