Answer:
Punnett square drawn below:
Step-by-step explanation:
Sorry if it's messy. Since brown coat is dominant to black coat, that would mean that the genotype for brown is either Aa or AA. Since it says it's a pure breeding brown mouse and that all of the offspring are brown, that means that the genotype for the pure breeding brown mouse is AA. As for the black coated mouse, black is reccessive, meaning the genotype would be aa. So... AA x aa = 100% Aa