The equation we can use is
3 + 1/4c = 9
To find the total number of pieces, we would just solve the equation.
3 + 1/4c = 9
3 - 3 +1/4c = 9 - 3
1/4c = 6
inverse of division is multiputcation
4/1 *1/4c = 6*4
c = 24
There were 24 pieces of cake in total.
Subtract 13 from both sides.
-13
-7 = -20
-2x = -20
Divide by 2 on both sides.
Remember: Negative divided by a negative makes a positive.
-2/-2 = 1
-20/-2 = 10
Your answer:
x = 10
When the slope isn't defined, it's not possible to write the line in slope-intercept form. No slope means a vertical line, which here must be

The ordered pair (-1,6) obviously satisfies the above equation so is on the vertical line
.
The answer will be
f(3)=3(3)+7
=9+7
=16