You need to find the value of a so solve the first equation in terms of x:

-3a + x = 3
x = 3 + 3a

then substitute the result (3 - -3a) into the second equation:

8a + 7(3 + 3a) = -7
8a + (7 x 3) + (7 x 3a) = -7
8a + 21 + 21a = -7
8a + 21a = -7 - 21
29a = -28
a = \( \frac{-28}{29} \)
a = -\(\frac{28}{29}\)

The surface area of a cube is (2 x length x width) + (2 x width x height) + (2 x length x height):

sa = 2lw + 2wh + 2lh
sa = (2 x 2 x 7) + (2 x 7 x 9) + (2 x 2 x 9)
sa = (28) + (126) + (36)
sa = 190

To solve this equation, isolate the variable for which you are solving (a) on one side of the equation and put everything else on the other side.

-3a + 5x = -9a - 4x + 7
-3a = -9a - 4x + 7 - 5x
-3a + 9a = -4x + 7 - 5x
6a = -9x + 7
a = \( \frac{-9x + 7}{6} \)
a = \( \frac{-9x}{6} \) + \( \frac{7}{6} \)
a = -1\(\frac{1}{2}\)x + 1\(\frac{1}{6}\)

The formula for area is πr²:

a = πr²
a = π(4²)
a = 16π

To multiply binomials, use the FOIL method. FOIL stands for First, Outside, Inside, Last and refers to the position of each term in the parentheses.