( 15u-21)*3+27=234 (36t+98)/14=25(12a+30a+42)/6=21

1) (15 * u - 21) * 3 + 27 = 234; 

Делим уравнение на 3. 

(15 * u - 21) * 3/3 + 27/3 = 234/3; 

15 * u - 21 + 9 = 78; 

15 * u - 12 = 78;

15 * u = 78 + 12;

15 * u = 90;

u = 90/15;

y = 30/5;

u = 5;

Ответ: u = 5.

2) (36 * t + 98)/14 = 25;

36 * t + 98 = 14 * 25;

36 * t = 14 * 25  - 98;

36 * t = 350 - 98; 

36 * t = 252;

t = 252/36;

t = 7;

Ответ: t = 7.

3) (12 * a + 30 * a + 42)/6 = 21; 

(a * (12 + 30) + 42)/6 = 21; 

(a * 42 + 42)/6 = 21; 

42 * (a + 1)/6 = 21; 

42/6 * (a + 1) = 21; 

7 * (a + 1) = 21; 

a + 1 = 21/7; 

a + 1 = 3; 

a = 3 - 1; 

a = 2; 

Ответ: а = 2.