From Mechanics

First consider the lower mass.

If \displaystyle T=T1 is the tension in the lower string then

\displaystyle mg=7\times 9\textrm{.}8=68\textrm{.}6\ \text{N}

giving \displaystyle T1=68\textrm{.}6\ \text{N}

If \displaystyle T=T2 is the tension in the upper string then the two masses are regarded as one particle with total mass 15 kg.

Thus in this case

\displaystyle mg=15\times 9\textrm{.}8=147\ \text{N}

giving \displaystyle T2=147\ \text{N}