According to questions
$l \propto \,{h^p}{c^q}{G^r}$
$l = k\,\,{h^p}$        …………..($i$)
Writting dimensions of physical quantities on both sides 
$\left[ {{M^0}L{T^0}} \right] = {\left[ {M{L^2}{T^{ – 1}}} \right]^p}{\left[ {L{T^{ – 1}}} \right]^q}{\left[ {{M^{ – 1}}{L^3}{T^{ – 2}}} \right]^r}$
Applying the principle of homogeneity of dimensions we get

$P – r = 0$         ………($ii$)

${2_p} + q + 3r = 1$        …………($iii$)

$ – P – q – 2r = 0$         …………….($iv$)
Solving eqns. ($ii$), ($iii$), and ($iv$), we get
$P = r = \frac{1}{2},q =  – \frac{3}{2}$
From eqn.$\left( i \right)\,l = \frac{{\sqrt {hG} }}{{_c3/2}}$