Multiplying Radicals: Where Is My Mistake?

[Cuberoot1]

I did this problem on paper but my calculator doesn't agree with the result. Can somebody tell me where I'm going wrong and how to do it right?

(7-sqrt3a)(7+sqrt3a)
= (7-sqrt3a)7+(7-sqrt3a)sqrt3a
= (7•7-7sqrt3a)+(7sqrt3a-sqrt3asqrt3a)
= 49-7sqrt3a+7sqrt3a-3a
= 49+3a-14sqrt3a

Even before I used the calculator something looked wrong, but I can't figure out how to fix it.

 

[evinda]

I did this problem on paper but my calculator doesn't agree with the result. Can somebody tell me where I'm going wrong and how to do it right?

(7-sqrt3a)(7+sqrt3a)
= (7-sqrt3a)7+(7-sqrt3a)sqrt3a
= (7•7-7sqrt3a)+(7sqrt3a-sqrt3asqrt3a)
= 49-7sqrt3a+7sqrt3a-3a
= 49+3a-14sqrt3a

Even before I used the calculator something looked wrong, but I can't figure out how to fix it.

(Wave)

$$(7-\sqrt{3a})(7+\sqrt{3a})=7 \cdot 7+ 7 \sqrt{3a}-7 \sqrt{3a}-(\sqrt{3a})^2=49-(\sqrt{3}a)^2$$
In general, it is known that $(a-b)(a+b)=a^2-b^2$.