Susan is 5 years older than her sister. the sum of their ages are 51. define a variable. then write an equation that could be used to find their ages

We will say the variable to be x.
So we will say her sister is x years old.
susan is x + 5 years old

     susan = x+5
     sister = x
 the equation wll be (x + 5) + x =51
ans = (x+5) + x = 51

if you want to continue
yo first collect the like terms wich are the variables that look alike and simplify them

in this case you take x and add to x making it 2x
then you cross 5 to the other side of the equal sign
then because t is positive it will turn to negative. then you will minus it wiy=th 51

in equation form it is done like this;
(x+5) + x = 51
x+x +5 = 51
2x + 5 = 51
          →
2x = 51 - 5
2x = 46
2x/2 =46/2
x = 23

if her sister is 23 years old then susan is 23 + 5 years old