Ax+8y=20 (4;7)

To find the value of the expression ax + 8y when x = 4 and y = 7, substitute these values into the equation.

ax + 8y = 20

Replacing x with 4 and y with 7, we have:

4a + 8 * 7 = 20

Simplifying, we get:

4a + 56 = 20

Subtracting 56 from both sides of the equation:

4a = 20 - 56

4a = -36

Dividing both sides by 4:

a = -36 / 4

a = -9

Therefore, when x = 4 and y = 7, ax + 8y is equal to -9(4) + 8(7), which equals -36 + 56, which equals 20.