3 + 0.75x = 5.25 - 0.25x
0.75x = 2.25 - 0.25x
x = 2.25
3 + 0.75(2.25) = 5.25 - 0.25(2.25)
3 + 1.6875 = 5.25 - 0.5625
4.6875 = 4.6875
They are equal
Well if its fora circle it would be r ( radios, Half of the Diameter) divided by 9
Using substitution:
first you have to express one variable in terms of the other, in this we can express y in terms of x:

Since both expressions are equal to y, you have to equal both expressions like this:

Now you can solve the equation:

Knowing x=10, you can use any of the expressions we found before to find y. In this case I'm going to use y= -x+9 because it's simpler but boy should give you the same result

So, the answer is x=10 and y=-1
f(x) = 20x² - 18x - 25
f(-1) = 20(-1)² - 18(-1) - 25
= 20 + 18 - 25
= 13