leniar equations in 2 variables

let the ten's digit be x and the unit's digit be y.

so the number is 10x+y

acc to question,

 10x+y=7(x+y)...(i)

and

 10x+y=3xy-12..(ii)

from (i) we get,

3x=6y

i.e.

 x=2y...(iii)

substituting this value of x in (ii) we get,

solving this quadratic, we get,

y=-1/2

 or y=4

 a digit can't be a fraction so y can't be -1/2

 so y must be equal to 4.

 so x=2y gives

 x=8

so the reqd num is 84.