Write down the equation on paper at first (I omit the "new" here):
A * x = b
[2 1 -1; [x1; [0;
0 2.5 -1.5; * x2; = -3;
0 0 2.4] x3] 4.8]
You have a triangle matrix on the left, such the back-substitution can be applied directly. Start to solve the last row to get x3:
0*x1 + 0*x2 + 2.4*x3 = 4.8
Use the result to solve the 2nd last row to get x2 and so on. A short look into WikiPedia might be useful also.
