your weight = your brother's weight + 3
And, your weight + your brother's weight = 155
And so let's say your brother's weight equals 'x' and your weight equals 'y'.
So, y = x + 3
y + x = 155
So we can substitute the first equation into the second:
y + x = 155
(x + 3) + x = 155
2x + 3 = 155
2x = 155 - 3
2x = 152
x = 76
And since your brother's weight equals 'x' , then his weight is 76kg