18 shells because since Mary has 6 and Tina has 3 shells all you have to do is multiply 6 and 3 to get 18
The double angle formula for the sine is

The right hand side is exactly your expression, where 
So, rewriting the expression from right to left, we have

Answer:
m = (p+7n)/3
Step-by-step explanation:
3m-7n = p
3m = p+7n
m = (p+7n)/3
Perimeter = (x+2) + x + (x+2) + x
Perimeter = 4x + 4
Area = x(x+2)
Area = x^2 + 2x