U can use the second last one
X + 90 = 2x + 40
I'm not exactly sure what you're asking but in total there's 800 kids at East Side Elementary School.
Given:
The equation of the curve is:

To find:
The gradient (slope) of the given curve at point (2,7).
Solution:
We have,

Differentiate the given equation with respect to x.


Now we need to find the value of this derivative at (2,7).




Therefore, the gradient (slope) of the given curve at point (2,7) is 19.
If 21 is added to (+ 21) a number (x) the result (=) is 40 less than (- 40) twice the number (2x)
x + 21 = 2x - 40 Add 40 to both sides
x + 61 = 2x Subtract x from both sides
61 = x Switch the sides to make it easier to read
x = 61
Sin C = 24/ 51
hope this helps