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If you mean by a(t)=2t+9 is the acceleration function then we integrate...

v(t)=2t^2/2+9t+c where c is vo and we are told that that is -7 so

v(t)=t^2+9t-7 integrating again we get:

s(t)=t^3/3+9t^2/2-7t+c where c is the initial position which is 8 so

s(t)=t^3/3+9t^2/2-7t+8  which neatened up as

s(t)=(2t^3+27t^2-42t+48)/6

Rate of change of velocity with respect to time is known as acceleration.

Position of particle is given by,  [tex]s(t)=\frac{t^{3} }{3}+\frac{9}{2}t^{2}-7t+8[/tex]

Given data are,   a(t) = 2t + 9, s(0) = 8, v(0) = −7

Since, Rate of change of velocity with respect to time is known as acceleration.

           [tex]a(t)=\frac{dv}{dt} \\\\a(t)dt=dv\\\\(2t+9)dt=dv[/tex]

Integrating both side.

We get,  [tex]v=t^{2}+9t+c[/tex]

Substituting v(0)= -7 in above equation

We get,   c = -7

So,  [tex]v=t^{2}+9t-7[/tex]

Since,  [tex]v= \frac{ds}{dt}\\\\ds=(t^{2}+9t-7 )dt[/tex]

    Integrating both side

We get,  [tex]s=\frac{t^{3} }{3}+\frac{9}{2}t^{2}-7t+c[/tex]

Substituting s(0)=8 in above equation.

 We get,   c = 8

So, position of particle becomes,

             [tex]s(t)=\frac{t^{3} }{3}+\frac{9}{2}t^{2}-7t+8[/tex]

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