Question 909148
let x =father's present age and y =daughter's present age
{{{x+y=42}}} - first equation

7 years later:
x+7 =father's age y+7=daughter's age
{{{x+7=3(y+7)}}}
{{{x+7=3y+21)}}} - second equation 

use either elimination or substitution on the two equations
by substitution:
{{{x=42-y}}}
{{{(42-y)+7-3y-21=0}}}
{{{4y=28}}}
{{{y=7}}}
{{{x+(7)=42}}}
{{{x=35}}}
Father's present age: 35