Answer:
![W(t) = 90 + 6t](https://img.qammunity.org/2021/formulas/mathematics/college/wam47gsvdopglphzupt642aw3478em0w63.png)
Explanation:
The weight W, related to the number of months t, is given by the following equation:
![W(t) = W(0) + at](https://img.qammunity.org/2021/formulas/mathematics/college/1jpud7ric0kjs74t1bf9f0p1avakq671l6.png)
In which W(0) is the initial weight and a is the slope.
Started at 90 kg
This means that
. So
![W(t) = 90 + at](https://img.qammunity.org/2021/formulas/mathematics/college/y6t5umkqqtt5z3rwkjcziq85ck889ie4fn.png)
Gained weight at a constant rate after eight months he weighed 138 kg
This means that
. So
![138 = 90 + 8a](https://img.qammunity.org/2021/formulas/mathematics/college/851gds2unetwfp5ti1wj7qdtecxeee9p4f.png)
![8a = 48](https://img.qammunity.org/2021/formulas/mathematics/college/i5ijfrr27ebg1rr3xj6aclorzcaamklcxn.png)
![a = (48)/(8)](https://img.qammunity.org/2021/formulas/mathematics/college/qgm5s02a9wq8qw7vljc374krfjmz2xf37b.png)
![a = 6](https://img.qammunity.org/2021/formulas/mathematics/middle-school/bh4a4uusxb4hfdexek1y9nnsqfo48p0xvm.png)
So
![W(t) = 90 + 6t](https://img.qammunity.org/2021/formulas/mathematics/college/wam47gsvdopglphzupt642aw3478em0w63.png)