1. **State the problem:** We are given the equation $(3 + \sqrt{a})(4 + \sqrt{a}) = 17 + k\sqrt{a}$ where $a$ and $k$ are positive integers. We need to find the values of $a$ and $k$.
2. **Expand the left side:** Use the distributive property (FOIL) to expand:
$$
(3 + \sqrt{a})(4 + \sqrt{a}) = 3 \times 4 + 3 \times \sqrt{a} + 4 \times \sqrt{a} + \sqrt{a} \times \sqrt{a}
$$
which simplifies to
$$
12 + 3\sqrt{a} + 4\sqrt{a} + a
$$
3. **Combine like terms:** The terms with $\sqrt{a}$ combine:
$$
12 + a + 7\sqrt{a}
$$
4. **Set equal to the right side:** The equation becomes
$$
12 + a + 7\sqrt{a} = 17 + k\sqrt{a}
$$
5. **Equate rational and irrational parts:** Since $a$ and $k$ are integers, the rational parts and the coefficients of $\sqrt{a}$ must be equal separately:
- Rational parts: $12 + a = 17$
- Irrational parts: $7 = k$
6. **Solve for $a$:**
$$
a = 17 - 12 = 5
$$
7. **Solve for $k$:**
$$
k = 7
$$
**Final answer:**
$$
a = 5, \quad k = 7
$$
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