A4. Two sides of a triangle are $7a-4b$ and $11a-3b$ units long, where $a$ and $b$ are natural numbers. What is the third side if the perimeter is $21a+2b$ units?
(A) $15a+16b$
(B) $2a-21b$
(C) $3a+9b$
(D) $2a+5b$
(E) $3a-9b$
A4. We use the formula for the perimeter of a triangle $o=a+b+c$, substitute the given data $21a+2b=7a-4b+11a-3b+c$. We calculate that $c=3a+9b$.