Question 661033
Write as an algebraic equation.

brother's age = x
David's age = x+3

The sum is 23 so:

x+(x+3)=23
2x+3=23
2x+3-3=23-3
2x+0=20
2x=20
2x*1/2=20*1/2
x*1=10
x=10

Check to see if 10 and 13 are solutions. 10+13 is 23.