=      (1)   (T = the teacher's age;  H = the husband's age)

T + H = 105.         (2)


First step is to multiply eq(1) by 6 (both sides) to simplify the system:

2T = 3H - 60         (1')
T  + H = 105.        (2')


Next step is to express  H = 105 - T  from (2') and then to substitute it into eq(1') to get

2T = 3*(105-T) - 60,

2T = 315 - 3T - 60,

2T + 3T = 315 - 60  ====>  5T = 255.


And now YOU make the last step.