We first need to remember that stress is force over area. We can calculate the area using the diameter as the specimen is told to be cylindrical (circular cross-section). The area can be calculated using 0.25 x pi x d^2 where d=1.5x10-4, the answer is 1.767x10^-8. Now we can re-arrange the formula for stress to make force the subject: this gives force equals stress multiplied by area. The stress at breaking is given so the answer is achieved by multiplying 1.767x10^-8 by 1.3 GPa (1.3x10^9) giving roughly 23N.