$\mathrm{T}_{r+1}={ }^4 \mathrm{C}_r\left(a \mathrm{x}^2\right)^{4-r} \cdot\left(\frac{70}{27 \mathrm{bx}}\right)^r$

$={ }^{+} \mathrm{C}_{\mathrm{r}} \cdot \mathrm{a}^{4-\tau} \cdot \frac{70^r}{(27 \mathrm{~b})^r} \mathrm{x}^{s-3 t}$

here $8-3 r=5$

$8-5=3 r \Rightarrow r=1$

$\therefore \text { coeff. }=4 a^3 \cdot \frac{70}{27 b}$

$T_{r+1}={ }^7 C_r(a x)^{7-r}\left(\frac{-1}{b x^2}\right)^r$

$={ }^7 C_r \cdot a^{7-r}\left(\frac{-1}{b}\right)^r \cdot x^{7-3 r}$

$7-3 r=-5 \Rightarrow 12=3 r \Rightarrow r=4$

coeff. : ${ }^7 C_4 \cdot a^3 \cdot\left(\frac{-1}{b}\right)^4=\frac{35 a^3}{b^4}$

now $\frac{35 a^5}{b^4}=\frac{280 a^3}{27 b}$

$b^3=\frac{35 \times 27}{280}=b=\frac{3}{2} \Rightarrow 2 b=3$