let x is her brother's age
Kayla's age is 3 less than twice her brothers age: 2x - 3
and
Kayla is 13
so
2x - 3 = 13
add 3 to both sides
2x - 3 + 3 = 13 + 3
simplify
2x = 16
divide both sides by 2
2x/2 = 16/2
x = 8
So her brother's age is 8 years old
proof:
2x - 3
= 2 (8) - 3
= 16 - 3
= 13