2  minus  the  sum  of  u  and  6? i just need the equation

To be crystal clear, I'd use parentheses here:

2 - (u + 6). If you wish to simplify that, you'd get 2 - 6 - u, or -4 - u.

Eventually you'll have to know how to do these problems yourself, so I'd suggest you stop writing, "I just need the equation." You need the know-how.

36 girls

step-by-step explanation:

the 48 boys is 4 parts of the ratio

divide 48 by 4 to obtain one part of the ratio

= 12 ← 1 part of the ratio

3 parts = 3 × 12 = 36 ← number of girls

v = 1/3 a^2h

step-by-step explanation:

the volume is 1/3 a^2h.