If you have p(-3), simply substitute it into the formula where x is:

Hence, p(x) = 8x + 9

so...... p(-3) = 8(-3) + 9 = -24 + 9 =-15

In general:

If you have p(something), substitute something into the formula where the variable which the formula is concerned with used to be:

e.g

f(x) = 2x^2 + x - 1

f(x+1) = 2(x+1)^2 + (x+1) - 1

f(x^2) = 2(x^2)^2 + (x^2) - 1

f(0) = 2(0)^2 + (0) - 1

f(4) = 2(4)^2 + (4) - 1

etcetc